Sepulcher

The Sepulcher is a post-Moon Lord boss servant enemy that is spawned during the fight with Supreme Calamitas. It spawns twice during the fight: Once when the initial bullet hell ends, and later when Supreme Calamitas reaches 8% health, shortly after the final bullet hell. Ten Brimstone Hearts will spawn along with it, distributed mostly in the upper corners of Supreme Calamitas's arena. These hearts must be destroyed in order to kill the Sepulcher.

The Sepulcher, notably, deals no contact damage, instead firing diagonal bursts of from its segments in order to damage the player.

Trivia

 * The word "sepulcher" refers to a kind of tomb or burial chamber.