Skeletron Prime

Skeletron Prime is a Hardmode mechanical boss which is essentially a more difficult version of Skeletron. It consists of a head and four limbs, but, similar to Skeletron, only the head needs to be defeated to defeat the boss.

Skeletron Prime can be manually summoned using a Mechanical Skull at night. It can also spawn naturally upon dusk with a chance of 10% once a Demon or Crimson Altar has been smashed, as long as it has not yet been defeated in the current world. If spawning naturally, its arrival is announced by the status message: "" ( alternatively "You feel the air getting colder around you..."). If it is not defeated before dawn, it will behave much like a Dungeon Guardian, Not using its limbs at all, and charging at the Player at extremely high speeds.

Attacks
Skeletron Prime has four limbs which are all immune to the Poisoned and On Fire! debuffs:
 * Prime Cannon fires bombs that do not destroy blocks, usually up into the air rather than directly at the player. It usually stays above the player.
 * Prime Saw usually remains under the player, moving up and down.
 * Prime Vice will swipe around wildly, making occasional lunges at the player. It usually remains diagonal to the player.
 * Prime Laser shoots red death lasers similar to those of Retinazer and The Destroyer, and is always pointed at a player. It usually stays horizontal to the player.

Skeletron Prime will occasionally enter a "rage mode", in which its attacks become more aggressive:
 * Skeletron Prime's Head will grow spikes and start spinning towards the player slowly. Damage and defense are doubled to /  and, respectively. The spinning speed is increased in Expert Mode (nearly the same speed with Dungeon Guardian).
 * Prime Cannon will aim at the player directly and fire rapidly.
 * Prime Saw will begin homing directly towards the player's location.
 * Prime Vice becomes more aggressive and frequently lunges towards the player.
 * Prime Laser will fire more frequently.
 * Skeletron Prime lets out a every time it enters rage mode.

Skeletron Prime is generally much more mobile than its predecessor Skeletron, and will regularly move off-screen. Like Skeletron, if it is not beaten by dawn, it will behave like a faster Dungeon Guardian, travelling at a speed of 65 instead of 41 MPH, and killing the player instantly if touched. If defeated this way he will drop items normally.

Tips

 * Standing on platforms far from blocks can mostly nullify the effects of the Prime Cannon, as its projectiles only detonate on blocks, while passing through platforms harmlessly, if not playing Master Mode
 * Unhindered movement is crucial when Skeletron Prime's head starts spinning, as it deals double the regular damage. Wings greatly aid in moving vertically, and flattened terrain or a sky bridge are convenient for horizontal movement.
 * Although it is a little more time-consuming, focusing on the arms first is very rewarding. Once Skeletron Prime's head is no longer shielded by anything, the fight gets a lot easier. Prioritize damage on the arm which is causing the most damage or disruption, so it can be destroyed and removed from the fight as quick as possible.
 * Due to Skeletron Prime's overall high defense, inflicting it with is effective, as he is not immune to it. Note that most Ichor weapons lack in damage compared to other weapons, so they should only be used periodically for inflicting the debuff.
 * In fact, as the head is Immune to Cursed Inferno, but not Ichor, Crimson World Players have an advantage over Corruption World Players.

Trivia

 * Skeletron Prime will open and shut its mouth for the entire battle. This is a purely cosmetic effect.
 * It is the only mechanical boss whose name contains the pre-Hardmode boss's name. Furthermore, it is the only mechanical boss to share boss music with its pre-Hardmode variant. Additionally, its name does not begin with the word "The", unlike the other two Mechanical Bosses.
 * Unlike Skeletron, Skeletron Prime's defense doubles when its head is spinning.
 * Skeletron Prime's head is immune to the Cursed Inferno, Penetrated, Celled, and Daybroken debuffs. Its arms, however, are not.
 * Normally, Skeletron Prime’s head has 2,000 less HP than Plantera, but in Expert Mode, they both have the same amount.

History
Skeletron Prime Grand Squeletron 机械骷髅王