The Eldar are synonymous with psychic might, so I figured I would cover psychic powers first. They have access to two unique psychic power tables: the Runes of Battle, which can be used by Spiritseers and Warlocks, and Runes of Fate, which can be used by Farseers. In addition, Farseers can use Divination and Telepathy while Spiritseers can use Telepathy.
Runes of Battle
The Runes of Battle have unparalleled flexibility, as each power can be use either to buff your own units or to hurt the enemy. This flexibility comes at a cost, as the buffs only work on the unit that contains the psyker. This isn’t a big deal for the Independent Character Spiritseer, but Warlocks can only join Warlock Councils and Guardian units, which don’t benefit from many of the powers. Fortunately, the offensive versions of the powers can target any enemy within 18”, making them useful even with Warlocks.
The primaris power is Conceal/Reveal, which either gives the psyker’s unit Shrouded or takes Stealth and Shrouded away from an enemy unit. This is a great primaris power, as Guardian units tend to hug cover, and it would give Jetbikes a standard 3+ Jink save for added durability. In addition, it can make enemy units that rely on cover saves much more vulnerable, and can be invaluable in dealing with flying Nurgle Daemon Princes, Stealth Suits, and Eldar Pathfinders. Fortunately, this is a pretty solid power for both the Spiritseer and Warlock.
Destructor/Renewer combines the old psychic heavy flamer with the ability to heal a wound on one friendly model. The heavy flamer can be handy for a jetbike character or member of a Warlock Council, but most of the time you won’t want to get that close to the enemy. As it’s a psychic power, it can’t be used for overwatch, which is where it would be most useful for Guardians. Renewer is most useful for putting wounds back on our tough single models, particularly Wraithknights, Wraithlords, and Pheonix Lords. They’re tough enough to kill without psykers healing their wounds.
Embolden is conceivably a useful power, as it makes the psyker’s unit Fearless. This will make sure they hold the line, but won’t add to their durability or killing power. The alternative power, Horrify, is pretty useful as it reduces an enemy unit’s Leadership by 3 points. This can help you break an important enemy unit either through shooting or assault, but will probably be most used in conjunction with the Eldar’s many pinning weapons (Rangers and Serpent Shields come to mind).
Enhance hasn’t changed and still gives the psyker’s unit +1 Weapon Skill and Initiative. It wasn’t used much before because Guardian units really shouldn’t be in close combat. However, it is a great addition to a Warlock Council, and the Spiritseer allows you to use it on a squad of Aspect Warriors or Wraiths. I think Wraithblades armed with ghost swords would benefit the most from this, but Striking Scorpions are also a tempting target. Fortunately, the alternative power, Drain, takes 1 point of Weapon Skill and Initiative away from an enemy unit. Since these traits are always compared relative to an enemy unit, dropping an enemy’s stats is the same as adding to your own unit. This means that you should almost always cast Drain rather than Enhance, as it has the same overall effect but can be used to influence any combat within 18”, not just one involving the psyker.
Protect/Jinx is probably the most exciting power on this list for me, as it allows you to improve the psyker’s unit’s armor save by 1 or drop an enemy unit’s armor save by 1. Defensively, this provides a small bonus for most Guardians, but Jetbikes gain a 2+ save and become really durable. On the other hand, the Spiritseer can make Wraithblades, Wraithguard, or Striking Scorpions a 2+ unit, which would be tremendously durable (I left out Warp Spiders as it will be difficult for the Spiritseer to keep up with them). Offensively, this is most useful for dropping enemy armor saves to the point that your weapons can bypass them. For example, if you dropped a Terminator unit’s armor save to a 3+, you could target it with the Fire Prism’s AP3 large blast and easily destroy it.
Empower/Enervate allow you to add a point of Strength to the psyker’s unit or take a point of Strength from an enemy unit. Once again, adding a point of Strength to any unit the Warlock can join isn’t much of a big deal; this even applies to the Warlock Council as they all have witchblades. However, it would be very handy for Wraithblades and Striking Scorpions. Enervate can be used defensively to weaken an enemy unit that’s about to assault you, hopefully keeping the generally squishy Eldar units alive a little longer.
Overall, the Runes of Battle are a useful set of powers. In every case, at least one of the alternatives will be useful in most circumstances. In general, I think the blessing version of each power is the weaker option as it can only affect the psyker’s unit, and the units that you generally put psykers with won’t benefit much from the many close combat oriented powers. The maledictions, on the other hand, look tremendously handy for both dulling the killing power of enemy units and making them more vulnerable.