Demon is generally a very stable talent and only a couple things seems to bug it. Occasionally using it on slopes will cause it not to deal damage due to a monster hitbox issue--Reaper appears to have a similar issue. (You may have noticed from using cone effects in the past that D:OS has a very difficult time with slopes, more often than not) Unfortunately this particular weakness appears to be completely out of our control.

Another thing that can happen is that Demon will get blocked by hidden LOS issues. Some objects in D:OS have larger effective movement and sight blocking boxes than their model would imply. Alternately, there are places in D:OS where the AI grid has been doctored that can cause a discrepancy between what you see as a player and what the games "sees" for targeting purposes. These issues are relatively rare, but I know that we inherited a couple from the vanilla game.

The final thing I have noticed is that we have seen a few reports about the cathedral in which your screenshot is taken. This structure appears to mess with reactions, attacks of opportunity, and Reaper, with no rhyme or reason as to when, why, or whom it affects. Despite thorough search, we were never able to find any clue as to why this area appears haunted.

If the animation for Demon appears, there is supposed to be damage accompanying it, because the animation and damage are, internally, produced by the same effect. As Elric touched upon, there is something odd about that area. My guess is that there's some sort of object blockage present there that prevents the damage from occuring correctly.

Sadly, there's not much to do about it--apologies :(

It works! That cathedral was the first fight I took after taking Demon, and the next fight it worked just fine. And it hit hard.

Good to hear! Demon is a lot of fun to play with. It always feels good to set up combos where your support characters get to hit enemies with a couple thousand fire damage just by casting a debuff.

By the way, I absolutely LOVE your responsiveness on this forum! I'm really, really enjoying this modificiation of what may be the best RPG I've ever played. The tactical element of the mod is a wholely fresh take on Divinity's combat system which I'm absolutely adoring- I'm likely to go through multiple full playthroughs of the game again. I'm even playing a multiplayer run with my friend simultaneously- we're both playing knights, so this will be exciting! We're running a demon knight and a paladin (soldier/pyro/witchcraft and soldier/hydro/geo). It's going to get much rougher as we progress further in the game, I think, but it's pretty hilarious to just THUNK people to death in unison! Amusingly enough, Empowered Splintering Arrow doesn't actually require a ranged weapon to wield, so I've been opening fights with casting Splintering Arrow off of my amulet; if I crit, I can almost one-shot a zombie since I'm using a high-level version of Sacrifice, the two-handed artifact sword.

Fantastic! We are very pleased to hear that you were able to work through your earlier difficulties and that you are now crushing content!

And yes, there are a couple of unadvertised gems like Splinter Arrow floating around that we left alone because they were interesting and would have required very odd workabouts to remove. This is only the "tip of the iceberg" as far as exotic builds and strategies are concerned. :D