I just killed the cultists demon and when I spoke to the dwarf, all he said was 'well done, I'll be on my way' or words to that effect. He didn't even give me any experience. Is this a bug or just the biggest anti-climax since the end of this sentence

Yeah, I'm afraid that's pretty much it. The reward for completing the quest is the EXP you get from killing Aec'Letec.

After such a tough battle, I had hoped for more, too. The XP would've been nice if I hadn't already hit the cap. ;)