An artifact mask Mask of Many Faces, neutrally aligned. When invoked, you are prompted to target an adjacent creature. If that creature fails a monster magic resistance check, or a d(its maxHP) roll is lower than its current HP, it is instantly killed, dropping its possessions but no corpse or other special drops, and the mask changes to that creature’s form.

Subsequently donning the mask, or having it worn when invoking it, merges it into your skin and polymorphs you into that monster’s form, starting a timer (not the usual polymorph timer) after which the mask will turn blank and the polymorph ends. Lower level monsters than the player will produce a longer timer, and higher level monsters and uniques will produce a shorter timer. You can also invoke it again while wearing it to end the polymorph early.