Slayer gates are challenging 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.
For those looking to open the slayer gate on Arc Complex, you can do it fairly early in the campaign mission. After you arrive at the front of the complex guarded by giant tentacles, you will need to use the turrets to destroy both of them and access the building to reach Doctor Hayden. The first location takes you on the right side of the map, and you’ll fight in a large building complex. You need to go through the building, fighting several waves of demons. You need to reach the portion of the building where you’re standing in front of a destroyed convention center entrance.
You need to go to the level where there is a Modbot you receive a modification for any of your available weapons. Behind it, you should see a hallway you can enter, and on the left is a staircase. Walk up to it, and you should be at the front of the slayer gate. Luckily, the key is not far away. In front of the slayer gate is an opening in the wall, and you want to double-jump to the right side where you can reach a grabbable wall. Climb up this wall, and turn around to face the slayer key, which you can achieve with a double-jump and a dash. With the key in your inventory, you can return to the slayer gate and complete it at your leisure.
There are six slayer gates found throughout the game. If you complete all of them, you can return to the Fortress of Doom to access the final weapon in the game.