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 left. However, you need to finish them all to complete the Demonic Corruption meter and to unlock a final weapon.
The first slayer gate occurs on Exultia. You won’t have too much trouble locating this key, but it might be a little easy to miss if you’re not paying attention. You find it shortly after you jump across several falling platforms, avoiding the lava underneath them and the flaming chains.
The slayer key and gate are right next to each, but you need the key to access it. To acquire the key, instead of proceeding through the arch next to the glowing green torch, jump up the various platforms to the highest one. You should see a dash canister floating in the middle of the room. You need to double jump, and then dash to grab the canister, and then use two more dashes to reach the other side. You will now have access to the slayer key, which is floating on a lower platform. The slayer gate is underneath you. Jump down to left and unlock it. Good luck attempting to complete the gate.
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.