I'm doing the Waking Nightmare quest. I've done it on previous playthroughs with no problem, so I know what to expect.

Warning: Spoilers ahead!

Everything was going fine until I got to the end, where Erandur and I are supposed to kill Veren and Thorek. The problem is, they won't die. It's like they're marked essential. Using the console to either kill or disable them has no effect. I reloaded earlier saves twice (from before leaving Dawnstar to walk to Nightcaller Temple), and that didn't help either. I even tried making a little mod in which they were definitely non-essential, but that didn't work either. I also tried using the console to advance the quest, but the only stage it will let me advance to at that point is 200 (i.e. the end of the quest); if I do that, Erandur never destroys the Skull of Corruption and Veren and Thorek just become non-hostile.

Has anyone else had this problem? Anyone know how to fix it? I'm at a loss.

I just finished that quest myself, and my only thought is that they are supposed to deliver dialogue lines before they engage the player and Erandur in combat. Is it possible that the game got interrupted or that you tried to attack them pre-dialogue or mid-dialogue somehow? I imagine this could cause a problem.

Hm, I've heard of Erandur glitching after you've killed them but never those two being unkillable. If there's any dialogue, let it play out before doing anything. That's a common problem with all quests if you attack/do something before NPCs finish talking.

Also, I'd try disabling all of your mods then giving it a go, see what happens then. If not, I'd try maybe using the console to reset the quest, though I'm not sure that'd work since reloading earlier saves didn't.

I've had this quest glitch out because of the missing dialogue as mentioned above. I reloaded saves about 3 times before it would work. As I recall I needed to make sure that I stood still for all of their little conversation while I was in the dream (past) realm. If they do not play that out fully they didn't work properly later after you run up and open the barrier. I had to give them a long time to make sure that they played their interaction right out.

I waited for them to finish their conversation and attack us. In the dream, I also waited until the conversation was over. Will try disabling mods.

If you really have no luck, you might try loading up a save from before any quest stuff was initiated. I cannot remember 100%, but think I may have had to load up a save from before I even entered the place to get it to work.