Much like others here, one of the closest encounters I had with an adventurer involved lots of !!FUN!!
In this case, my adventurer just had a hard-earned conquest over a tower, newly trained in the arts of necromancy. Just as he was sneaking out of there, the wilderness around him suddenly combusts in flames. The fat from his body was already melting.
Preparing for his funeral, I decided to survey the area to see just what caused this. Patches of land were already ablaze in smoke. Looking carefully, I noticed a creature that I've never seen wandering the wilderness; a demon. I didn't know it at the time, but it must have come from a cave not far from the tower.
I thought to test the might of this fiend first-hand. I had him run back to the tower, and as he did, he stopped melting, fortunately enough. Putting his newfound power over the dead to good use, I had him recruit some fresh meatshields, and finally pursued the beast in better terms (from a place that wasn't on fire).
The monster, of course, begins wrecking havok upon the undead. My adventurer then shot at the beast with bow and arrows from ambush. To my surprise, despite this adventurer being a legendary archer and bowman, the beast dodged all shots handily. Worse still, the monster was now blowing flames right at the adventurer, and though none of the shots connected, it did manage to light the ground on fire.
So now the adventurer's fat was melting again. I tried to have him flee, but the pain he was in slowed him down significantly. I looked around for some bodies to raise between him and the beast to buy time, but could only find smears of grease and rubble. As a hail Mary, he raised some distant corpses, hoping the monster would be redirected there. Through a miracle, this plan worked perfectly. This gave the adventurer time to run off to sneaking distance, live through the melting fat, and fast travel back into town. But the injuries had a dramatic impact on his movement speed, so I let him retire quietly.