Skeletron Prime

. "You feel the air getting colder around you..."

Skeletron Prime is a hardmode version of the Skeletron boss. He can be summoned with the Mechanical Skull at night and players have until dawn to kill him. He can also randomly spawn at dusk if he has not been defeated yet, in this case his approach is signalled by "You feel the air getting colder around you" or "The air is getting colder around you".

Skeletron Prime has four limbs, each of which functions differently. The Prime Cannon fires flashing bombs that do not destroy blocks 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 and the Prime Vice swipe around, dealing large amounts of melee damage.

The four limbs are immune to the Poisoned and On Fire! debuffs, while the head is also immune to the Cursed Inferno debuff.

Skeletron Prime performs a headspin attack analogous to his lesser version. During this, his damage and defense double, unlike Skeletron whose defense reduces to 0. The Prime Saw homes in at players during this phase, even going hundreds of feet away in some cases. The Prime Vice, however, does not hone in during this phase.

Like Skeletron, if he is not dead by dawn he will begin to dish out instant kills and will take only 1 damage (2 for crits) from all attacks.

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.