This page of our guide to Doom Eternal explains how to reach Slayer Gate 5, which is located in the Mars Core level.
How can I open Slayer Gate in the Mars Core level?
Use the switch to move the wall in the next room. Now you can jump up, right into the cracked ventilation shaft. You'll find a key to Slayer Gate there. Enter the gate and confront another waves of monsters.
Slayer Gate Battle 5 - Mars Core level
You've got a tough ride ahead of you. You will be greeted by a Marauder and Unmaykr creatures that are supported by weaker demons. Focus your firepower on Unmaykr creatures and kill them with headshots. Also, prepare Super Shotgun and the Ballista to kill Marauder - shoot at the enemy only when you will see a green light.
The second round is consisting of lots of Hell Knights and Barons of Hell. Move all the time, use ice bombs and BFG, if you have any plasma cells, to quickly get rid of many enemies. After the fight, you will receive another Empyrean key.