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.
You can find a slayer gate near the end of the Cultist Base mission. It’s right before you use the train to enter the Terrordome to kill one of the priests. You find it in the room where you need to knock down the giant statue to break a hole through the wall and use it as a bridge to the next room. You want to knock down the statue first before going after the slayer gate.
After you knock down the statue, look to pit to the left of it. You should see a wall you can grab if you dropdown. Do so, and grab the wall. Look behind your position to see a yellow bar. Double jump to it, but hold your dash. After you swing on the pole, use your jump to grab the wall on the other side of it. You should not have to use your dash to reach these two locations, but it’s a useful safety net if you need it. Turn around at your current position, and you should see a dash canister. To reach the other side of the wall, you will need to use your dash, and the canister, to reach it.
Once there, climb up the wall and enter the upper platform. You need to grab the yellow and to swing up even further, and the slayer key should be right there available for you.
The slayer gate is not far. The gate originally blocking you from the key should open, allowing you to reenter the room where you smashed the statue. Turn to the right, behind the block you used to raise the floor to reveal the jump boost, and you can access the slayer gate. Good luck fighting it.
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.