"The fallen overseer drives his smaller impish charges into a frenzy with his ape-like leaping and growling. An overseer among a group of fallen is a dangerous thing, as the cowardice that usually characterizes them is overwhelmed by fear of their much larger brethren."— Zoltun Kulle(src)
Overseers are often found amidst their smaller impish brethren. In combat, they drive the latter into a frenzy with the ape-like leaping and growling. They are encountered in the second and third acts of the game.
The weapon that they wield resembles two old swords attached parallel to each other to a large, mace-like stick. In addition to normal attack, they can also make a 'charged' leaping strike that deals greater damage and Knockback.