Collect all six Empyrean Keys from the six Slayer Gates to unlock the secret Unmaykr weapon in the Fortress Of Doom. The gun case can be found by going to the center chamber and dropping down to the lowest level, then go forward to the tall chamber. Empyrean Keys drop from Slayer Gates. Slayer Gates are very difficult combat encounters found in multiple missions throughout the game. Check the maps on each mission for a purple "Slayer Gate". To unlock a Slayer Gate, find a nearby Slayer Key. The Slayer Gates are well-marked after finding the Auto-Map. If you are having trouble completing Slayer Gates, you can replay any mission from the mission selection menu. Just return when you are more powerful. The Unmaykr first appeared in DOOM 64 as a secret weapon. You could find the Unmaker (or Laser Gun) in secret levels, and upgrade it into an amazing gun by collecting hidden keys. It is an incredible weapon, and was originally planned for the classic DOOM.