Note that this article is intended for competitive play and all that entails - the general consensus is that this codex is best used in combination with another to utilize its full potential and mitigate its weaknesses.
I have been playing the new codex since it dropped and it’s amazing. Currently I am running an Ultramarine battalion, auxiliary detachment consisting of a Culexus assassin and a Vanguard detachment for the Custodes. There are other good choices for the combination - Astra Militarum is obvious. I feel that the Culexus is a great choice to combine with Custodes since they do not have as much psychic protection as some other armies. Strategic placement of the assassin can go a long way to helping protect Custodes from Smite and psychic debuffs. I run Tigurius in my Ultramarines detachment who is also very helpful as well. Whatever you choose to combine with your Custodes should also provide some protection as well.
The basic stats for a Custodian are as follows:
BS2-WS2-S5-T5-3+W-A3+
Each and every Custodian is the equivalent of an HQ and they have an excellent arsenal to choose from for their weapons.
So now I’ll discuss my current choices for my Vanguard detachment and they are as follows:
HQ
Shield Captain in Allarus Terminator Armor
+Castellan Axe - Ballistus Grenade Launcher - Misericordia
This HQ is a true beast in melee swinging at S8 AP2 D3 damage and has seven wounds. He can also teleport allowing you to bring him in where he is most needed. I typically wait until the 2nd or third turn for his drop. He has beaten Celestine in protracted turns of melee and managed to walk away which says something about his survivability. Both he and the Allarus combine well with the Vexillus Praetor using a stratagem to drop one terminator unit within 6" of the Praetor and up to 3" from enemy units at the end of your movement phase. This is a guaranteed charge and either unit will wreck face. Note that Captains have a buff that allows other Custodes units within 6" to re roll hit rolls of 1 so he rarely ever misses.
Shield Captain on Dawneagle Jetbike
+Hurricane Bolter - Auric Aquilius (Relic - 3++ invulnerable save)
This Captain is the speed daemon quickly able to react to whatever the situation at hand requires. He has the same stats as the other Shield Captain except that he’s T6 and wields a lance (S6) in melee. He is the most survivable with the relic for the 3++ invulnerable save and extra point of toughness. As hard as both Shield Captains are neither is immortal either so don’t get overconfident how you use them.
Elites
Vexillus Praetor
+Guardian Spear - Vexilla Defensor
My Praetor has the Vexilla that grants a 5++ invulnerable save from enemy shooting to all Imperial bike and infantry units wholly within 9". This is excellent for a Space Marine gun line. There is also a stratagem that extends the range of the Vexilla an additional 6" For one turn. He is a character and has five wounds. Keep him safely tucked away so that evil things like daemon princes can’t consolidate into him.
3x Allarus Custodian
+Castellan Axe - Ballistus Grenade Launcher
These are the terminator clad Custodians wielding axes with four attacks and wounds each plus they can teleport. They will quickly make short work of anything in close combat. There’s a stratagem that causes their grenade launchers to lower enemy units’ BS and WS by -1 when hit plus they can’t fire Overwatch that turn. So for each terminator you can prevent one enemy unit from Overwatch. They can also use another stratagem to split up into individual units overwhelming your opponent for a turn. Note that these strategems while very powerful require a considerable number of command points so use them wisely.
3x Warden
+Castellan Axe - Misericordia
Wardens are an elite Guardian with a built-in FNP save and come equipped with Misericordia (one additional S5 AP2 attack in melee). They also have access to the Castellan Axe which in my opinion is overall superior to both the Sentinel Blade, Lance or Guardian Spear. There is a strategem that allows you to deep strike one unit which is what I typically use for my Wardens... the more units you hold in deep strike reserve the better, more so especially for units forced to foot slog.
I am not currently running the Vertus Praetors jetbike squad but you could use an Outrider detachment if you prefer this unit over the elite choices I’ve covered above. Vertus Praetors cost a bit more points but not that much more. Another detachment that is popular now is Supreme Command which lets you field three Shield Captains on dawneagle jetbikes. I prefer weight of numbers though which is part of the reasons for my choice of Custodian units.
One a final note if you should decide to select a Shield Captain as your Warlord there is a trait that gives him FNP and there is yet another excellent strategem that allows him to reroll one hit, wound or save per turn.