Skeletron Prime

. Skeletron Prime is a hardmode version of the Skeletron boss. He can be summoned with the Mechanical Skull. Players have until dawn to kill him; like Skeletron, when the sun rises, he will begin to deal out instant kills and will take only 1 damage from all attacks.

Unlike Skeletron, Skeletron Prime has four hands, each of which functions differently. The Prime Cannon fires bombs and isn't always pointed at a player. The Prime Laser shoots Death Lasers similar to those of Retinazer and The Destroyer, and is always pointed at a player. The Prime Saw swipes back and forth dealing large melee damage and homes in on players (even going hundreds of feet away in some cases) during the headspin phase. The Prime Vice also swipes around but doesn't home in during headspins.

Video Example
http://www.youtube.com/watch?v=gPMB-a6UGLA&feature=relmfu