Phase mist is an extremely rare phenomenon. It appears as a hazy cloud of swirling colors similar to those found in the Astral Sea. When a creature steps into phase mist, its body phases into the strange spaces that exist between the planes.
Effect: A square of phase mist is lightly obscured. A creature that starts its turn in a square of phase mist gains phasing until the end of its turn.
Usage: Use phase mist to create encounters that force the characters to split up. For example, the characters might use the mist to enter a chamber that lacks a door. Within the chamber is another patch of phase mist and an object that the characters need. In addition, monsters that have phasing, or undying guardians that lurk in the inaccessible chambers, wait to ambush lone characters.
Published in Dungeon Master's Guide 2, page(s) 61.