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Their primary stats are Willpower and Cunning. Most effects are based off Mindpower, which is affected by both Willpower and Cunning (with Willpower having a larger effect). Some talents have a Willpower prerequisite, others have a Cunning prerequisite.
Your most effective talent for directly hurting things at first is Willful Strike. This also knocks the monster back a tiny bit, which is certainly welcome.
Later, Reproach starts to become more effective. Emphasize Willpower, Willful Strike and Reproach during the early game. With Madness, Reproach lets you debuff a whole bunch of nearby monsters. In one-on-one situations, Reproach's damage actually starts to become respectable with decent Willpower and additional points invested. Its downfall is its incredibly short range.
Reproach does not hurt any allied NPCs, despite what the description implies. It only targets enemies.
For crowd control, you have Reproach (with or without Madness), Creeping Darkness (highly unpredictable -- it doesn't hit every square in the area you specify), and Blast.
Deflection's description is absolutely worthless. Here's how it works: you turn it on once (sustained), and it stays on forever. Each turn that you aren't damaged, it builds up a point (on the left pane, it will appear as Deflection (26), then Deflection (27), and so on until it has reached its maximum damage absorption based on the talent level and your Willpower). When it absorbs incoming damage, these points are reduced, and it will begin charging again. It's not a particularly strong shield, but it's extremely low-maintenance, and certainly better than nothing.
The Doomed do not fare well against strong enemy spellcasters. Taking antimagic may help with this -- believe it or not, the Doomed are eligible for antimagic, since Hate is actually a perverted form of psionics -- but then that leaves you needing to defeat a strong enemy spellcaster. Catch 22 situation.
Creeping Darkness may help prevent enemy spellcasters from seeing you until you get close.
A yeek doomed can cast feed (talent) on enemies, and then cast dominant will on them to regain hate without risk. This is epecially useful on molds/jellies which replicate on death, as the feed beam will continue to target the spawned copy.