[Taqwus]

Sure, there's blood slave sites. Most common non-national is Damned Merchant, +1 per. One of the Academies, maybe the one of High Magic, also gives +1, I think. Mictlan obviously has higher-volume one in EA.

Doom Horrors, like the Eater of the Dead, might have some undesirable hardcoding -- the going-independent. Not sure it's good for the game if the AI is betrayed by its pretender running amok and slaughtering its worshippers. Not sure whether Call God would work, either. Weird scenario. Definitely don't use Kurgi; the AI won't handle the feebleminding (bad when the Ashen Empire is completely dependent on mages), and you need Kurgi around to punish people who cast globals during Astral Corruption.

A humanoid lich-type or a Harbinger with the Sceptre of Dark Regency is very thematic, 'tho. Even a Prince of Death with the Sceptre of Dark Regency, the Crown of Katafagus or Amon Hotep would be vicious; classic fear + awe, and Amon Hotep gives MR to boot making it a bit harder to kill.

A well-equipped Mandaha is less thematic unless the theme is 'Unholy Alliance' between demons and undead, but the automatic darkness would be vicious and it's unique to Mandahas, if memory serves. Add the Black Book of Secrets for seasoning. I'd suggest the Sceptre of Corruption except that's horror-marking.

Tartarians are more thematic, being undead; the male Tartarian Titan, provided that it has no afflictions (are the afflictions added by the spell, or an inherent property of the unit?). Obvious weapon is Tartarian Chains. Maybe Tome of the Lower Planes as well, very appropriate. Consider using a cave fortress...

Etimmu's an obvious candidate for usurper, but you can't prevent him from showing up as a hero without mods. You could make the hero insta-drown by giving Ermor an -underwater- province, but you'd also lose Ermor-the-Pretender's immortality, the rest of Ermor's heroes, et al.

Again, fill slots. The AI will happily use horror-marking items like Lightless Lanterns. OK on sage, bad on pretender.