RARE KILL

Occasionally, you'll earn bonus experience for a rare kill.  This means the
poor thing you killed hasn't been killed in a long time.

This feature is to encourage exploration and reward beating up on things off
the beaten path instead of just doing the same ol' thing.  (Not that there's
anything wrong with the same ol' thing.)

The longer something isn't killed, the more exp it will build up (to a point).

See Also: EXPERIENCE