Cave Slime

This thin, blue slime is harmless but extremely slick.

    Effect: A creature that enters a square filled with cave slime must succeed on an easy Acrobatics check or fall prone.
    Usage: Use cave slime to shield artillery monsters from the characters. Skirmisher monsters that can fly or that excel at Acrobatics might dart back and forth across a patch of cave slime, defying adventurers to follow them.

Published in Dungeon Master's Kit, page(s) 210, Dungeon Master's Guide, page(s) 67.