I know nothing about D'Tesh, I'd never heard of it. I'm far from a Warhammer canon expert. If you read the first post of this thread though, you'll see we're planning to use something like the Scions Reborn mechanic too; where some units can turn defeated enemies into units that can be settled for more population.
Do you think we'd need something more than that?
The Legions of D'tesh are in a module, and also in FFPlus. The D'Teshi are a straight undead army with no living units...really...just like the Scions, except they are more lich based. Everytime they capture or raze a city they gain slaves they can add to their population or use as workers. The D'Teshi can also move these slave units between cities to distribute the population. I think the spell they use is called Call Slave or something. Removes 1 pop from the city and produces a slave unit. Most, if not all their units have a chance to capture units and turn them into slaves.
I like both the Scion and D'Tesh fallow civ models but I think D'Tesh would fit better flavor and mechanics wise. If you are not including the Scion's chance to spawn units that can add to pop then I think you should just use the D'Tesh work instead. D'Teshi also spread their own hell terrain, something which was discussed earlier. IMHO the D'Teshi would be worth looking at, and possibly simpler to use.
http://forums.civfanatics.com/showthread.php?t=276614