A monster that’s an evil blacksmith (cyclops?) with a special attack that applies harmful object properties to your equipment. They may also do this to items they find while roaming around the level, but leaving them on the floor rather than picking them up so it looks like nothing has been done to them.

Applying a negative object property could also be a monster spell, but this may not work as well as long as all arcane spellcasters use the same spell list; it would probably result in this effect being too abundant.