When used, Slaughter EX will hurt any nearby entity (excluding passive NPCs, invulnerable entities and the player themselves) and shine a light in the area. Depending on what bosses are killed in the world it's used in, it will:
Slaughter EX also takes away 50 life from the player who used it, after which it will inflict Silenced on said player for 10 seconds. It cannot be used if the player's life is below 200 or if the world that the player is in is not on Expert mode.
It is particularly useful for dealing with large groups of enemies, as the attack damage is enough to kill most enemies almost instantly, especially in the late-game.
It isn't recommended to use Slaughter EX during boss battles unless the player has a lot of skill, as it drains life.
However, if used correctly, it can be used to instantly defeat The Destroyer in one use. The player needs skill and speed in order to do this.
The same conditions apply if one wishes to use this strategy to instantly kill Eater of Worlds.