Overlords are the might heroes of Dungeon.
"Overlords build dungeon lairs to protect their gains acquired through conquest. Ruling through intimidation, they tend to be warriors who know the value of magic. They often wear armor designed to enhance the ferocity of their appearance." RoE manual
As they are heroes of an evil town, this allows them to sacrifice only creatures for experience on Altar of Sacrifice and in Grotto .
Overlords cannot learn Necromancy and Water Magic, which means they can only acquire them from Witch Huts (if allowed), Scholars, Seer's Huts, Pandora's Boxes or Events.