There are a lot of secrets packed away in DOOM Eternal including statues, music, lore, and secret encounters. To access these Slayer Gates, you will need to find a Slayer Key hidden somewhere throughout a level. Each mission has its own Slayer Key, so it’s critical you hunt them down if you want to access that weapon in Doom Slayer’s ship. Once you’re in the Cultist Base, fight your way almost to the end of the level where a giant monster is nailed to a cross.
Normally, you will need to climb up to punch a box that opens up a gravity lift. Instead, approach the left side of the statue and jump down to the left. You should see a wall that Doom Slayer can grab onto. Once attached, turn and jump to the yellow monkey bar and follow the path underneath. You’ll eventually have to make two back-to-back dashes so you can hit the Dash Reset power-up. This will let you dash two more times so you can reach the climbable wall.
Ascend the wall and jump into the room above you to claim the Slayer Key. This one is pretty easy to miss since there are no additional secret areas in this room and the Slayer Gate is tucked away. No just approach the door and unlock it to access the Slayer Gate! You might want to come back, however, as there’s a pretty tough fight right up ahead that has a bunch of high-level demons.