Necropharus
Located in several places:
Drefia - Underground

Drefia - Boss invasion in the Medusa Shield mission room.

Goroma - Lich Hell

Edron

As for the time to regenerate Necropharus, it's difficult to say because there are several places available and you can kill them after completing the task. I'm sure it appears at least once a week. It is possible that up to three a week.