Artifact — Equipment, 2 (2)
Uncommon
Equipped creature gets +1/+2 and has hexproof. (It can't be the target of spells or abilities your opponents control.)
Equip {3}
It hides the face and protects the soul.
Illus. James Paick

9/22/2011: If Mask of Avacyn somehow becomes attached to a creature an opponent controls, that creature can't be the target of spells or abilities you control.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Extended (Type 1.X)
Legal in Standard (Type 2)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern