Skeletron Prime

The Skeletron Prime is a hardmode version of the Skeletron boss, which was introduced in the 1.1 patch.

Unlike Skeletron it has four hands with different attacks for each. Skeletron Prime's four hands seem to have different designs. His eyes are red and his mouth has a chomping animation and when he enters the mode where his head spins, his head grows spikes. The blood animation on the normal Skeletron is replaced with fire on Skeletron Prime.

You can summon Skeleton Prime with the Mechanical Skull.