[Uh-Nu-Buh]

Seems like it might ADD a level of horror marking. They would note you as having killed one of their own. OTOH, you would certainly come away with something--a random thing:

1. fear/awe aura--something about you has changed that marks you among lesser men.
2. You gain insight into the universe from your encounter--random magic at 1 level.
3. You found an item on the horror--random item.
4. You took a piece of it as a souvenir (an extra horror mark, plus another pick on this chart).
5. During the battle, some of the Horror's ichor soaked into your own wounds--vastly increasing one of your attributes (e.g. ap, strength, protection, def lvl, mr)
6. Two more rolls on the table.