Slayer gates are extremely difficult encounters in Doom: Eternal. In them, you get transported to another reality where you fight of a seemingly endless wave of demons using what ammunition and extra lives you have on your person. If you don’t have enough, you reload to the next checkpoint, and sometimes you emerge from them with barely any bullets lets. However, you need to finish them all to complete the Demonic Corruption meter and to unlock a final weapon.
You can access the slayer key close to the end of the mission when you acquire all three of the keys. Of the trio, you need the blue key. When you have the blue key, make your way back to the large, circle arena that had the pillar of flesh in the middle, with teeth protecting a jump boost on either side, which required a blue key to access. When you have the blue key, follow your pathway to this point and open the teeth. Go through the area and use the jump boost to go into the air.
While in the air, turn and look at the flesh wall, which you can grab onto. Follow the path to the left and you will fight a Spectre, the invisible Pinkies. Kill it, and at the end of the pathway is the slayer key. Add it to your collection of keys, and this places you right overtop of the slayer gate’s location, which is to the left of you.
There are six slayer keys and gates in Doom: Eternal. You need them to unlock a hidden weapon on the Fortress of Doom. You can repeat these encounters when going through mission select, but you will not get another key at the end. You cannot access or acquire slayer keys or gates if you play a mission with cheat codes turned on.