This post will be part 1 of 2 in exploring how to build and integrate a CSM/Demon army with this first part covering Chaos Demons as the Primary detachment with CSM as allies with a sample list. The second part will cover CSM as the Primary with Chaos Demons as allies and another sample list.

The dilema:

Since Demons and CSM are redone for 6th edition and there are so many new possibilities that have poured out of the warp for us in regards to army composition, how do we build our armies, much less combine them with others? Successfully integrating Chaos Space Marines and Chaos Demons leaves us with some big decisions indeed... Questions such as "what units should we take," and, "do we make Demons the primary detachment so we can use 4 Heralds and risk the Warpstorm chart," or "do we make CSM the primary detachment to have multiple Hell Drakes and Cult Marines for troops?" Since I can't have it all, I'll explore the pros and cons of each option (Demons or CSM for primary) and go from there. I will hopefully help you with those choices as I take you through my own decision and thought process in juggling these very questions, so read on, enjoy, and post any comments or questions you might have!


Disclaimer #1: For starters, I am primarily a Tzeench and Thousand Sons player so this post will be focused on this style of army/play, however, the overarching concepts should apply to any demon army with CSM. 

Demons as the primary detachment:
The choice to take Demons as the primary detachment is one that would be a rather big change for me as I've always looked at Thousand Sons as the main army with Demons just tagging along to throw around as fodder. Indeed, Demons should be the thralls of the Thousand Sons and not the other way around! However, making Demons as the primary detachment opens up some very potent tactical options and doesn't put too much of a damper on the Legion's army...

Pros of using Demons as a Primary Detachment:
For starters, if Demons are taken as a primary detachment, you have more access to a wider range of units and more slots to take them in. This means that you have more tactical flexibility in making a list with Demons in the form of more and better choices. Taking Demons as Primary gives you the following advantages:

-Access to multiple HQs:
First off, you are not limited to either only 1 HQ choice as you would be if demons were only an allied detachment to CSM. This means more flexibility as a primary detachment since Demons have access to Demon Princes, Greater Demons and up to 4 Heralds (and only max of 4 heralds) for a potential total of 5 HQs. Compared to the CSM max of 2 HQs and you can quickly see how the Demon HQs are in a better position to support the rest of the army as you can spread them out in more units.

-Access to multiple heralds:
You can take up to 4 Heralds if Demons are your primary detachment which is a huge incentive because Tzeench Heralds get access to 3 Divination powers for dirt cheap and the powers can really buff CSM in a big way. Specifically (1) Prescience (primaris power) greatly increases shooting and close combat potential of all Demon/CSM units; (2) Forewarning (#2 power) gives units with the Mark of Tzeench a 3+ invulnerable save since Mark of Tzeench in CSM is +1 invulnerable save; (3) Misfortune (#3 power) gives your whole army a vastly increased chance of doing damage to a single target; (4) Perfect Timing (#4 power) makes the Tzeench Psychic powers incredibly lethal to models that rely on cover saves such as Eldar Pathfinders or Harlequins; (5) Scrier's Gaze (#6 power) is nice because you have a greater chance of rolling some very nice objectives such as the "Skyfire Nexus" or "Grav Wave Generator" in addition to getting your Hell Drake in sooner... Heralds also greatly enhance other Demonic units like nothing else as will be discussed below.

-Access to multiple units of Flamers/Screamers:
I'll just lump these guys all together. Sure they took a hit to the face with the nerf bat but I think it was necessary. Nobody should be able to win simply because they spammed a few units that are so powerful that all you need to do to win is move forward and roll dice (though there are some 5th edition armies that can still accomplish this).  Specifically, having the option for multiple units of these guys allows you to take the full allowable amount and do a lot of damage per unit, or, you can now take multiple smaller units. No more just having one unit of such and such as you would be limited to doing if Demons were simply an allied detachment. Perhaps a few units of 4-6 would be ideal, rather than just one big unit of 6-9 as I usually take if Demons were used as allies. Plus, since the nerf, you might need more of these guys to get the same effect. Also, a Herald can join either of these units and while a Flamer unit doesn't benefit much from a Herald, a Screamer unit will in that the Herald can tag along on a Disc and add some CC punch with some AP2 attacks and Divination powers.

-Access to cheaper troops:
Now all Demon troops are much cheaper. Sure you will need more Demons for troops (2 compulsory as Primary vs. 1 as allied) but as they are cheaper, it's not that big of a loss. You end up getting more for a similar cost (10 horrors cost 170 before, now 10 horrors costs 90). Anyway, since I focus on Tzeench and Thousand Sons, Horrors are a natural troop choice for the Primary Detachment so I'll discuss them in greater detail with their strong points up here in the "pros" section and their weak points in the "cons" section below. I will also touch on the Tzeentch discipline in the Demon codex since it is the only source of offense for the Horrors.

Good things about Horrors:
Horrors make for a decent choice because they are relatively cheap and they have the "Brotherhood of Sorcerers" rule which means they count as a Mastery level 1 Psyker and will be able to generate one power from the Tzeentch Discipline. This Psychic power that you generate is the unit's only form of offense so this is what you have to look forward to in terms of Horror offensive capabilities:

The Discipline of how fickle chaos is Tzeentch:
-Primaris power (Flickering Fire of Tzeentch):
A 24" S5 Ap4 2d6 shots Witchfire with the Warpflame and Soulblaze special rule. This is the only power that can increase in effectiveness from the additional Warp Charges that Horrors can get based on their unit size because for each Warp Charge beyond the first, you get an additional D6 shots. If taking a large unit of Horrors, you might want to default to this power every time as you will likely place a Herald in here and that gives the unit all kinds of benefits as I'll discuss below. Worth mentioning is a Flying Demon Prince or Lord of Change with this power can take shots at the rear armor of flyers and hit on a 2+ because of their superb BS which is something to consider.

-1-2 power (Tzeentch's Firestorm):
A 24" Sd6+1 Ap- Assault 1 Blast with the Warpflame rule. This is the "darn, I got the worst power in the game, time to take the Primaris power" power...What else can I say, it's a terrible power and I don't even know why they bothered with it when the Primaris is infinity times better.

-3-4 power (Bolt of Change):
A 24" Sd6+4 AP2 beam with the Warpflame rule. This is a great power and if I could, I would just spam Horrors with this ability all day long. On average rolls you'll be S7-8 or higher with this Beam which is great at hitting multiple vehicles or models. The AP 2 gives a nice +1 on the vehicle damage chart if you penetrate one in addition to negating Terminator armor.

-5-6 power (Infernal Gateway):
An 18" Sd6+4 AP1 Blast with the Warpflame rule. This is a dangerous power to get because if you ever get below 11 horrors (thus only having one Warp Charge) you can no longer cast this power (since it requires 2 Warp Charges). Otherwise, its a great power and if a Herald rolls it I might keep it because it is even better at taking out multiple vehicles with a nice scatter as AP1 will help destroy any vehicle you punch or it could put an end to any naughty terminators that Deepstrike near you and are in a neat little circle asking for this power to be cast at them.

The Horror's psychic might explored:
With that said, I like the S4+d6 Ap2 beam or the AP1 blast for anti-tank/TEQ capabilities but on Horrors, I think they were designed to take the Primaris power and with a Herald, that power becomes quite nasty.  Furthermore, because Horrors are Tzeentch demons, they get +3 to their Ld when casting powers so they are Ld 10 for psychic tests and they can get +1 to the strength of psychic powers from a Herald's Exalted Locus of Conjuration (making the primary a nice 2d6 S6 Ap4). Speaking of Heralds, Horror units get incredibly potent with a Herald in them so that is a huge plus as I can't see Horrors being very effective (or nearly as effective) without one. Heralds and Horrors go together like oxygen tanks on scuba divers, it's simply a must take combo. Putting this into perspective, consider the difference...

No Herald with the Tzeench Primaris power means simply 2d6+x S5 AP4 shots (x=an additional d6 for each additional warp charge you want to use, depends on the unit size). Adding a Herald means 2d6+x S6 AP4 shots from the Horrors plus the Herald's own powers wich could be the Tzeench Primaris power as well (note the +1 S from the Herald's locus). Also consider the ability to grant Prescience to the Horrors (allowing them to re-roll misses) and the fact that S6 can threaten many vehicles whereas their S4 shots in the previous codex couldn't even tickle the majority of vehicles. The sheer volume of shots combined with Prescience makes them ok at taking out flyers as well, especially if you get a Skyfire Nexus from rolling 3d6 for mysterious objectives (thanks to the #6 Divination power). That is a lot of firepower, as long as nobody makes their Deny the Witch rolls.

The other Tzeentch powers also benefit a lot from the +1 Strength and if possible, throwing out 2 of those powers (one from the Horrors and another from the Herald) makes for a potent anti tank/Terminator unit. I don't count the 1-2 power in this analysis as that one should always be dropped for the Primaris.

So with that said, taking Horrors in a Demon army that is your Primary Detachment gives you the ability to spam Tzeentch powers and when combined with Heralds, they are on par with guided War Walkers. Again, Just hope nobody passes their Deny the Witch roll...Because "Chaos is fickle."

-Access to Multiple force org slots:
As I mentioned above, you have access to multiple force org slots that allow you to have more flexibility in general. You can take more combinations of things with Demons as your Primary detachment where you would otherwise be limited to just one choice from a particular force org slot if you took Demons as an ally. Specifically, you can build an army that has Heralds, a Greater Demon and 3 Demon Princes if you wanted, or take 3 Soul Grinders and we all know that taking multiples of something increases the success of that something to do what you want them to do.

-Access to Hellforged Artefacts:
While you can get Hellforged Artefacts even if Demons are the Allied Detachment, I still think it's worth mentioning how they benefit both armies here...

The Eternal Blade is a very nice killy weapon for any Greater Demon/Demon Prince. What's not to love with an additional D3 WS/I/A? Probably overkill in most cases and you are probably better off with the other Artefacts as they will have a greater tactical impact on your games. If you really need the +D3 stats to win a combat, you probably charged the wrong target or are dead anyway.

The Portalglyph is probably one of the better ones as on a 4+ you get another scoring unit. Just triggering it once and getting around 3 models gets your points back and it provides you with so many options. Specifically, I like the idea of spawning D6 horrors since you only need 1 to cast their Psychic Powers and 2d6 S5 Ap4 shots are always nice. Its problems are (1) that it's an immobile vehicle with AV 12/12/12 so it may be relatively easy to destroy; (2) it scatters 4d6" because "Chaos is fickle" so it may end up being in a bad spot; (3) you need to roll a 4+ to activate it (because "Chaos is fickle"), so if dice are not in your favor, its a wasted investment; and, (4) if it is destroyed, it is potentially an easy Victory Point in missions like Purge the Allien or it might give First Blood. Its benefits are that (1) you can potentially get 6 free scoring units if you roll well enough (because "Chaos is fickle"); (2) it is only the size of a blast template so it can be hidden relatively well and at least get cover; (3) it scatters like a Drop Pod so it will never mishap and this type of scatter can be manipulated to prevent it from potentially scattering at all.  Just make a ring with your infantry, place portal glyph inside the ring and watch it stop 1" from a model... You can even sing "ring around the Portalglyph" while you set this up and rejoice as you overcame the authors' intent to show you how fickle Chaos can be with this item.

The Doomstone is interesting but not one that I'd ever take because it really won't provide much impact in the game the way I build my armies. First, because "Chaos is fickle," you need to be in combat with an enemy character without the Demon special rule for it to even come into play. Second, because "Chaos is fickle," you need to have the enemy Characters fail a Ld test. Third, because "Chaos is [more] fickle" than you realized, their Ld is only reduced by D6 and this may not make a difference in the combat/game at all since you still use the highest Ld in the unit for break tests. Fourth, as if Chaos wasn't fickle enough, a character will need to fail this test multiple times to potentially be removed as a casualty from this Artefact and by then, you are likely either dead or already won the combat in which case the enemy characters are all dead anyway. I can see using this Artefact being pulled off quite nicely, however, if you take IG allies and hit a squad with the Psyker Battle Squad's power Weaken Resolve then charge said squad with your Demon character equipped with The Doomstone...It's pretty hard to pass a Ld2 test and roll under a 1 on a D6, despite how fickle Chaos can be!

The Grimoire of True Names is an interesting option with CSM allies because you have the potential to give a unit a 2+ invulnerable save. Possessed, Warp Talons, Obliterators are all Demons with access to the Mark of Tzeench which gives them +1 to their Invulnerable saves so their 5+ Demon save goes to 4+ for Mark of Tzeench and that would further go to a 2+ from the Grimoire of True Names...I guess giving all of your guys name tags really makes them try harder, or something? Another possibility is using it on a Demon of Tzeench who has the #2 Divination power cast on them (4+ invul save) thus giving them a 2+ invul save if this item successfully activates...Since Tzeench demons re-roll 1s on saves, you basically have a re-rollable 2++. It would suck if you failed to roll that 3+ though, reducing their Invulnerable save instead...Chaos is Fickle.

Cons of using Demons as a Primary Detachment:

-Randomness:
Chaos is fickle, I get it...What isn't in 40k? The Randomness of the codex will be an annoying factor. First, the WarpStorm table can cause a lot of problems between potentially doing a lot of damage to both your army and your opponent's army to having dumb results such as removing one of your models or an enemy model to a failed 3d6 Ld test (which I always seem to fail). Second, not reliably getting certain desirable gifts because you need to roll for them is a hassle too. Third, some gifts, while awesome, need to be rolled for to even work (such as Portalglyph, Grimoire). At least you have some nice options with the default gifts and the greater artefacts can at least be guaranteed. Still, it's annoying if you want to get any one gift in particular like the S8 AP1 shot to give to a flying MC so he can get a nice rear shot on an enemy Flyer or if you need the Portalglyph to spawn that last troop's choice to get an objective but you fail to roll a 4+... Oh well, it is what it is but it's definitely a con to have so much random flying around.

-Very limited options from CSM codex:
perhaps one of the biggest cons (aside from the randomness) for taking a Demon army as the Primary detachment if you want CSM as allies is the limited number of options/slots you have with the CSM codex. Specifically, cult units cannot be unlocked as troops and you only have access to one Helldrake. This translates to a higher overal cost if you want a cult unit because you need a troop unit (that invariably can't be the cult unit) on top of the cult unit. Also, the cult unit must now come from the Elites slot which means you can't take any Terminators. Overall, if you don't mind having just one Helldrake and one cult unit (that isn't scoring while eating up an elites choice), then this isn't a big deal but if you are like me and would like to take multiple cult units, it's a real bummer.

-Horrors are a hit or miss troop choice:
While Horrors have a lot of pros as I listed above, they really have a lot of issues that warrant discussing down here in the lowly "cons" side. For starters, they have no natural shooting attack. Their only offensive capability is in the form of psychic powers cast through their Brotherhood of Psykers rule which means no overwatch. This is a shame because the #1 Divination power (overwatch on full BS and counter-attack) combined with Prescience (re-roll hits) would have made this unit quite strong against enemy charges if they still had their S4 Ap4 Assault 3 weapons...But alas, I think they changed it because that was probably broken if combined with the #3 Divination power (enemy re-rolls successful saves) because not only would Horrors erase everything from the board in their shooting phase, they would also never be successfully charged due to wiping out chargers in range (but oh how I wish the above were an option). Anyway, since their only offense is a psychic attack, a successful deny the witch roll/Rune Priest roll/Runes of Warding will nullify the 90-180+herald point investment for but a fraction of the cost.


Sample army list:


Demons as Primary: 1206

HQ: 475
Herald of Tzeench#1: 45+50(+2 Mastery)+25(disk)+20(rewards)=140
Herald of Tzeench#2: 45+50(+2 Mastery)+25(exalted locus of conjuration)=120
Herald of Tzeench#3: 45+50(+2 Mastery)+25(exalted locus of conjuration)=120
Herald of Tzeench#4: 45+50(+2 Mastery)=95

Explanation:
Herald#1 goes with a unit of Screamers and hopes for a combination of the #2, 3, 5 and Primaris powers from Divination to buff the Screamers with potentially 4+ invulnerable, re-rolls to hit and or forcing the intended charge target to re-roll all saves thus greatly increasing the potency of a Screamer unit. The demonic gifts are there to supplement any CC ability in the form of a Tzeench weapon or Etherblade. You can always add a greater gift of Chaos to try to get the Grimoire and hope for a 2++ or 3++ save on the unit.

Heralds #3/4 go with units of Horrors to buff the overall offensive capability and will choose a Tzeench  power and 2 Divination ones guaranteeing Prescience for each Herald along with a hopefully good Divination power like #2,3 or 4.

Herald #4 goes wherever you want, just take 3 Divination rolls to try and make sure you get everything you want from that discipline or just take a Tzeench power and place him in Horror unit with another Herald.

This provides the army with some good buffs from Divination and a good solid psychic offense/defensive base since Mastery 3 means denying most other psykers on a 4+.

Elites: 207

Flamers: 23*9=207

Explanation:
This is just a solid unit, regardless of the Warpfire rules. Essentially, you are supposed to shoot a target until it is dead dead dead so the bonus to regeneration won't matter. 9 Flamer templates at S4 AP4 will do a ton of damage to anything short of vehicles and high T models, especially if the target has had the #3 Divination power cast on it. This is a solid horde control unit and I still like them.

Troops: 324

Horror unit #1: 9(base)*18=162+x(if you want to take any icons/instruments or whatever you can)

Horror unit #2: 9(base)*18=162+x(if you want to take any icons/instruments or whatever you can)

Explanation:
Placing a Herald or two in each unit of Horrors greatly enhances their Psychic potential and offensive capability as discussed in the "pros" section. You can always keep the units smaller and do 2 units of 10 but since Demons are the primary and you have Heralds, why not go big? Up to 4d6 S6 Ap4 shots from the Horrors and 3-4d6 S6 Ap4 Shots from the Herald is A LOT of firepower, on par with Eldar War Walkers but more survivable...

Fast attack: 200

Screamers: 25*8=200

Explanation: Screamers are still a very solid and versatile unit. They can do some decent damage with their flyby attacks, get a ton of attacks at S4 and can get 2 (2 because you only exchange all of your base attacks and this does not negate the +1 from charging) S5 AP2 Armorbane attacks on the charge  for when you need to punch Meq/Terminator or Vehicles in the face. Put in a Herald and the unit is re-rolling misses and can potentially get a 4+ invul or what have you depending on the powers rolled for. The extra attacks at a better AP or S from the gift rolls from the Herald don't hurt either.

Heavy Support:?

Nothing here unless you have the points for a Soul Grinder or whatever else you want. I'd dedicate a Soul Grinder to whatever, hide him in cover and enjoy the Skyfire Autocannon or buy him a nice template and go to town.

Chaos Space Marine allies: 825

HQ: 110

Sorcerer: 60+50(+2 Mastery)=110

Explanation:
No need for any mark since you can't unlock Thousand Sons as troops in an allied detachment. This allows you to roll for 3 Telepathy powers and hope for Invisibility to cast on the Screamers (2+ cover anyone?).

Elites: 242

Thousand Sons: 150+92(+4 sons)=242

Explanation:
The only place you can fit some Thousand Sons as an allied detachment is in the Elites slot so here they go. Casting Prescience on them will make sure they put the hurt on those naughty Space Marines for wearing their silly Power Armor. Also, the #2 Divination power will greatly enhance the survivability of this unit (3+ invul). Conversely, if you don't care about having Thousand Sons, then you could always take a large unit of Terminators to benefit from prescience/#2 power and go to town on the enemy. I'd do both if I could =(.

Troops: 78

Cultists: 50+28(+7 cultists)=78

Explanation:
A cheap and largish scoring unit that can fill the troops choice and a good place to stick your sorcerer.

Fast Attack: 170

Trogdor Hell Drake: 170

Explanation: Burninate the country side....

Heavy Support: 225

Obliterators:70*3=210+15(Mark of tzeench)=225

Explanation:  Nothing like some Obliterators firing some Las Cannons, Plasma Cannons, Assault Cannons or Cannon Cannons of Cannoning into some juicy targets, right? Don't forget the Prescience! You can also change the mark of Tzeench to Nurgle for T5 as it adds a lot more in terms of protecting the unit from S8-9 weapons.

Army list overview:
This comes out to be a total of  1206 for the Demons and 825 for the CSM for a total of 2031 so you can shave some points off from the Horrors (or wherever else you want) to drop it down to under 2k or drop the horrors down to 10ish each and drop an Obliterator to make the list under 1850. I think the list has a little bit of everything: decent CC from herald/Screamers, Obliterators and the Thousand Sons/Sorcerer aren't exactly pushovers in combat with Prescience up;  Great shooting against all kinds of targets and while only 3 scoring units, they are decently survivable and can be protected by the other elements of the army. I would consider moving some things around/dropping Herald #4 for a Lord of Change with a Portal Glyph (he costs 285) so you can have another potent CC unit with the ability to spawn more troops.


Conclusion:
Some difficult decisions indeed, and I both commend and curse GW for forcing me into this dilema. First, it makes me think harder about how I want to play an army and second, it keeps me from having it all and sadly, we can't always have it all, can we? How will you guys combine Demons and CSM, if at all?