Skeletron Prime

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

Unlike Skeletron it has four hands with different attacks for each. Skeletron Prime's 4 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.