Green Slime

Green Slime is a thick greenish goo of arcane origin that seems to spontaneously grow within dark, damp environments. It converts any material that comes into contact with it to more Green Slime, except for earth and stone. Fire quickly destroys Green Slime, causing it to boil away into a foul but harmless vapor. Additionally, if Green Slime is exposed to direct Sunlight for at least 1 hour, it evaporates.

When a creature comes into contact with Green Slime, it must succeed a Saving Throw [5d10 vs CON] within 10 segments or it is dissolved into Green Slime. If fire is applied to the Green Slime, the creature may make a Saving Throw as a Free Action, reducing the difficulty of the Saving Throw by 2d10 [STB 2d10].

While not sentient, Green Slime has the bizarre tendency to grow mostly on ceilings and drip onto creatures as they walk underneath.