The Tesseract

The Tesseract is a Hardmode tool that is dropped from the Lunatic Cultist. It eliminates all enemies on the screen at the cost of 200 mana and 10% of the player's health, as long as the player has above 25% health, and the enemy is not a boss or has over 3000 max health.

Trivia

 * Tesseract is the term for interdimensional teleportation in the book "A Wrinkle in Time". Maybe the enemies are being teleported to other dimensions.
 * The actual definition of a Tesseract is a 4 dimensional figure, though in “A Wrinkle In Time”, which this is likely a reference to, it is a 5 dimensional object,