Santa-NK1

Santa-NK1 (also known as Santank) is a boss that makes an appearance during the Frost Moon. It acts similar to Mourning Wood, often chasing the target before stopping after a time to unleash one of three attacks.

When below half health, the face and beard of the Santa mask will be torn apart to reveal a rather deathly skull underneath, one oddly familiar... that of Skeletron Prime.

Trivia

 * "Santa-NK1" is actually a play on the word "Santank", the name used when the Santa-NK1 was announced. Santa-NK1 -> Santa-nk1 -> Santank1 -> Santank
 * Santa-NK1 might be a reference to Santa Claus in Futurama.