A Slayer Gate is a gate (requiring a "Slayer Key") controlling access to an area containing a special purple gore nest. If the player pulls the organ out of the nest and succeeds in defeating all demons in the ensuing fight, the player receives three weapon points, the nest explodes, and an Empyrean Key is left in its stead. Six Empyrean Keys are required to unlock the Unmaykr in the Fortress of Doom.
The reason for the existence of the Slayer Gates and how they teleport the player to a different environment are not explained in-game. The environments within the Slayer Gates largely resemble the surroundings of Night Sentinel architecture, in particular, areas which are parts of Exultia, Sentinel Prime, and Taras Nabad. The inside area behind the gates themselves also strongly resemble Sentinel Prime's architecture.
List of Slayer Gates
Slayer Gates are found in these levels: