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Using Peace of Mind pyro buff on the rat also clears its affliction. gets its a mental debuff...
A way to help the Historian without an elf and blood rain that worked for me: Attack one of your party members until there is blood on the ground, teleport the Historian on the blood, bless it, throw a poison bomb ect. on him so he doesn't die.
I teleported one of my characters in front of him and attacked it with other person, the blood splattered on historian and doused the flames
didnt get the quest
So long story short, I have a possible bug for this quest, details are contained below. If anyone can figure out what went wrong, it should probably be made note of.

Does going around to the back before even starting the quest potentially bug it out? Or might it be something else I've done? I went around the back way when I was exploring the island and walked up to the door of the tower, was told I entered the wrong way, then cleared out the tower. Afterwards I went around to the front end of the maze, and the quest started as normal. I got quest updates as I made progress, though admittedly I hadn't used a single skull by the time I made it to the Historian, and I had all but two of them by that point. Finished helping the Historian, got the last remaining skulls, went to the door of the tower, and nothing happens. No dialogue or anything. The quest was still marked as active though. So I reload my save, the quest updates after using two skulls, I use a third, then I try the gargoyle at the tower door again. Still no dialogue or quest update. So I'm not sure where I went wrong, but the fact that I'm getting no dialogue and the quest is unable to be completed seems like it's probably some kind of bug.

Hopefully reloading from before I start the maze and just doing it over will fix it, but I feel like that should be made note of, if anyone has any idea what went wrong. (Also, I do not have Braccus's ring, if that matters at all. I flubbed that up by not immediately breaking Trompdoy's soul jar and just stuffing it into by bag.)
Same damn problem here. The Gargoyle didin't activate,k even after reloading save with no skulls picked.
To put out the Hellfire condition on the Historian.
After my fight in that area, there was a cloud of steam nearby. I teleported the Historian into that cloud and it caused him to stop being on fire. After that I could Bless him.