Apostoles of Armok, the caste, are extremly underpowered. Basically if you get one on embark, you're fucked. Reasoning:
They only get a 200% bonus to one skill, praying, and cannot get any xp in any other skill. That's cool and all, but praying is one of the worthless skills. The entire religion thing is really well balanced, the issue is leveling up your praying skill is good for one thing and one thing only: getting better crafts from sacrificing blood/warlock items so that you can trade, and faster extraction of admantine. Can you see hte problem? If you get an apostole of armok as one of one of your embarking things, he's absolute dead weight. It's even worse because you can't even increase the one skill he's good at AT EMBARK.
Suggestions to fix Apostoles of Armok,
1. First, merge the mage and apostoles of armok guild. All apostoles of armok should also be mages, and learn socery easier. That brand new magic system might work well here. To compensate, you could cut down learning praying a bit...but really, not learning any other skill makes it unneccesary
2. Open back up the medical tree to these. give them a slight boost (like 25% faster). They by no means replace the healers guides, but priest and healers were synonymous back in the ages. If not that, then at least let the poor bastards learn crutch walking.
3. Scribe. let it be one of the other skills they learn. Hell, make it another thing their proficient at. remove it from the craftdwarves guild, it doesn't make much sense there anyways.
4. Playing music. Why not. Let them learn this, at least. Why not, its not like it's going to hurt.
5. Runesmith. Erm. hello? Apostole of Armok. dwarven runes. I think the connection between magically enhancing materials with dwarven runes and Armok is pretty clear.
6. They should get a boost to learning bookkeeping and organizing. It's a bloody church.
7. finally, and this is a big one: an apostole of armok should have a reaction that has a much higher chance of getting food/drink/water at the shrine of armok, and also have other misc reactions for the lowest tier shrine that gives some useful benefit.