Santa-NK1 (also known as Santank) is a mini-boss that spawns during the Frost Moonevent. When below half health, the face and beard of the Santa mask will be torn apart to reveal a rather morbid skull underneath.
Santa-NK1 repeatedly cycles through three attacks:
First attack: Chain gun fires bullets at the player.
Second attack: Presents launch high, then rain down via parachute from above, causing damage on contact with players. These travel through blocks.
Third attack: Rocket barrage. These also travel through blocks.
While moving, it also drops large spiked balls on the ground that deal high damage (120-150+ with Chlorophyte armor). These increase in frequency as Santa-NK1 takes damage.
Notes
The drop rate linearly increases from wave 13 (or 15) to 20.
Santa-NK1 currently has one of the highest damage rates in the game.
Trivia
"Santa-NK1" is actually a play on the word "Santank" (ie. Santa Tank), the name used when the enemy was announced.
Its arm appears to be the Chain Gun, which it sometimes drops.
The Santa-NK1, while not very visually similar aside from a mechanical nature and tank treads, behaves very similarly to a recurring mini-boss from the freeware Apogee game Bio Menace. The enemy, called the "Robo-Tank", shares similar attacking patterns such as multiple rapid-fire heavy ordnance weapons like missiles and bullets. Most specifically, the Robo-Tank also fires a volley of bombs into the air, which must parachute down onto the battlefield.
It is rumored that Santa-NK1 is Skeletron Prime due to the face looking like Skeletron Prime's skull.
History
1.2.3.1: Its Projectiles are fired slower and do less damage.