Farseer
The Farseer is the iconic Eldar HQ, a psyker designed to augment the rest of the army around him. As a level 3 psyker with access to the Runes of Fate, Divination, and Telepathy, the Farseer is very capable in this role. Otherwise, he doesn’t do much. With his witchblade, he can do a few reliable wound in combat but he won’t kill much. Armed with a singing spear, he can add some short range shooting to a squad and reliably damage vehicles. With a 4+ invulnerable save, he’s not easy to kill but not too tough either.
On the table, the Farseer just needs to stay alive and within range to use his powers. The best way to do this used to be to give him a jetbike. However, I don’t think that’s so necessary now that his powers have a much longer range. If you have points left over, Runes of Witnessing would be useful to make sure he passes a critical psychic check. I’m not so sure about the Runes of Warding; they seem very situational. As far as the Remnants of Glory, only the Spirit Stone of Anath’lan really benefits the Fareseer. It allows him to reduce the cost of any power by 1, to a minimum of 1. This is pretty useful given the number of Warp Charge 2 powers in the Runes of Fate. However, using it means the Farseer can’t use his invulnerable save until the start of his next turn. That’s a pretty hefty penalty, but if you can protect the Farseer, it may be worth it.
Overall, the Farseer is a great addition to any Eldar list. He primarily aids a shooty list, but it’s 6th Edition Eldar; of course you’re playing a shooty list. Keep him out of direct danger and he’ll make your army much more efficient.
Spiritseer
The Spiritseer could be considered a junior Farseer, but that’s not really fair to him. However, his statline matches that since he’s lacking a wound and point of LD compared to the venerable Farseer. That’s pretty major, especially for a psyker. Speaking of that, he’s also only a level 2 psyker. This isn’t too big a deal, as the Runes of Battle powers all only require 1 warp charge. He also is a bit cheaper than the Farseer.
Despite these similarities, I think the role of the Spiritseer is very different. The Runes of Battle are much more geared to supporting the unit he joins and debuffing the enemy, and he could be used to create a legitimate Eldar death star. The only other way to access these powers is through a Warlock, and they are very limited in which units they can join. Beyond his powers, he also makes Wraithguard and Wraithblades Troops rather than Elites.
Combined, all of these factors mean that the Spiritseer is a respectable HQ choice in a variety of lists. If you have a central infantry unit, he can make it much stronger and tougher. He can give you really tough scoring units; this is worthwhile even if you’re only taking one squad rather than an Iyanden style army. The Spiritseer can bring a variety of benefits to an Eldar army that no other HQ can.
Warlock Council
The Warlock Council has been changed up a bit. It still doesn’t take up a FOC, but you can only take one per primary detachment. You take all of your Warlocks as a unit, and they can be split off to join any Guardian based squad at the beginning of the game, similar to Wolf Guard or the Necron Royal Court. Somewhat unfortunately, this happens before they roll their psychic powers, meaning that you can’t send Warlocks to squads that would benefit most from their specific power.
Warlocks come with rune armor, shuriken pistols, and witchblades, and can take singing spears and jetbikes if you want. Unlike last edition, they operate just like normal psykers. However, they’re mastery level 1 and LD 8, meaning that their powers can be pretty unreliable. They have access to the same Runes of Battle as a Spiritseer.
I see Warlocks as most useful for supporting Guardian squads. Many of their powers benefit standard Guardians, and they can also act as a distributed source of maledictions to weaken your opponent. Alternatively, you could take a large Warlock Council and keep it together as an assault unit. Like always, this is more viable when you give them all jetbikes. I’m not sure how viable this build is, as assault tends to be weaker now and they won’t reliably have Fortune anymore. On the other hand, they have the chance to get a 2+ armor save; if they get Fortuned too, that could be ridiculous. Even their primaris, Conceal, gives them a 3+ cover save as long as they move.
Eldrad Ulthran
Eldrad has been toned down a bit, but he’s still the best individual psyker the Eldar have. He’s mastery level 4, and has a chance to regain a warp charge every time he passes a psychic test. However, he can no longer cast any of his powers twice in the same turn.
Beyond his psychic abilities, Eldrad comes with 3+ save rune armor and a AP3, Force singing spear. He allows you to redeploy D3+1 units just before the game, though they have to redeploy in your deployment zone, so they can’t be moved in or out of reserve. He also has the warlord power that gives every unit within 12” Stealth for a single turn. Given how squishy Eldar are, that can really add some resilience to your army for a critical turn. It’s particularly handy to survive a nasty alpha strike and give you a chance to return fire.
Eldrad is no longer an auto-take choice. While he’s got some handy rules, his psychic might is outclassed by taking two standard Farseers, and at a similar point cost. He certainly is good enough for his points to be a viable choice, but you’ll do just as well with standard Farseers or Spiritseers. I see Eldrad being used mostly for theme lists, or for lists where you want to take a non-psyker HQ (such as the Avatar or a Pheonix Lord) but you still want strong psychic support. Eldrad brings unparalleled psychic power if you only have one HQ slot available.