Skeletron Prime

. Skeletron Prime is a hardmode version of the Skeletron boss. He can be summoned with the Mechanical Skull or has a very small chance of spawning at a random time (although still not sure whether this is a bug or not). 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 (2 for crits) from all attacks. It is possible to survive this form with only Spectre Boots and Wings by flying in an infinity figure. (You will need to continuously move.) His approach is signalled by "You feel the air getting colder around you" or "The air is getting colder around you".

Unlike Skeletron, Skeletron Prime has four hands, each of which functions differently. The Prime Cannon fires grenades 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.

Trivia

 * The Prime part of his name may be a reference to Optimus Prime from Transformers, the "Prime" suffix having become common for large robotic entities in fiction since. It may also mean the highest rank since Skeletron Prime is like an older brother or elder of Skeletron.