The Mask of the Skull is the wondrous item which has, through a single use, caused me the most grief as a DM. During one adventure, the party traveled to the depths of a volcanic dungeon, finally arriving at the lair of an ancient guardian red dragon. During the first round of combat, the last player to act before the dragon loosed his Mask of the Skull against the dragon, defeating it before it even had a chance to defend itself properly. To this day I loathe the mention of the Mask.

Use of the Mask of the Skull is quite simple. Once per day, remove the mask from your face and throw it at your target. The exact rolls needed are found in the Dungeon Master's Guide, but if you succeed, the target dies as per the finger of death spell. Many epic creatures and undead are immune to death effects, but the item proves useful even against certain powerful foes like dragons. Remember to pack one next time you foresee a draconic foe, and perhaps you, too, can draw the ire of your DM.

-The Wizard
"C'mon, man, put the mask down. That is so not funny." -- The Wizard while DMing, after the described event.