Covetous

Certain high-level monsters are covetous, in that they will try to steal one of the Unique items, or your quest artifact. The quest artifacts of other roles are immune from this theft, as is yours if it is stashed in a bag. There is no defence against this theft for items in your main inventory; this is especially troublesome if your only source of magic resistance is your quest artifact.

On one-twentieth of the hits from such an attack, the monster will look for one of the above items in your inventory; if it finds one it will steal it and, if capable, teleport away.

The Wizard of Yendor is most notorious for this, as if he double troubles his double has a chance of carrying a cheap plastic imitation of the Amulet of Yendor which can be confused with the real amulet if it is stolen.