After so many kills the Avatar of the creature comes. For example: Someone
is camping the Highhold orcs for 7 hours, looking for that particular item.
The party has killed NUMEROUS orc fanatics, each one a servant to the orcs
god. The god notices the death of 'its' subject. The god sends an avatar to
destroy those that are killing his worshippers. In comes The Shralok Avatar.
It takes names. It kicks butt. If the party zones, he will disappear until
anyone from that party comes back to that area. After a certain length of
time, it is safe to come back and camp the area some more to get another of
a particular item. Until the avatar shows up again.

Its only a suggestion :)