An abyssal wellspring is a suppurating wound in reality. It is a place where the raw madness and evil of the Abyss bulges into another plane.
Effect: An abyssal wellspring appears as an area of at least 4 contiguous squares. When a creature starts its turn in a wellspring square, it takes 5 damage per tier, and gains a +1 bonus to attack rolls per tier and a +2 bonus to damage rolls per tier. These bonuses last until the end of its turn.
Usage: One obvious thematic way to use an abyssal wellspring in an encounter is to pair it with demons, especially brutes like bariguras. They have enough hit points to take the damage from the wellspring, and the extra bonuses give them a nastier punch that normal. A wellspring can also offer an interesting choice to characters when you pair it with higher-level soldier monsters with high defenses. The characters must decide if the bonuses to hit and damage are worth the extra damage taken.
Published in Manual of the Planes, page(s) 21.