The following page of our guide to Doom Eternal explains how to get the last Empyrean key which is hidden behind Slayer Gate available in Taras Nabad level.
How can I open the Slayer Gate in Taras Nabad level?
To get the last Empyrean key, you have to dive into the water. Destroy the wall while being underwater, then shoot the button to reach the next wall - remember to lower the water level. Go through the open gate, but instead of following the marker, head to the other side of the tunnel to reach another Slayer key.
Slayer Gate Battle 6 - Taras Nabad level
You will have to confront the following monsters - Arch-vile, Pain Elementals and Arachnotrons. Focus all your firepower on Arch-vile as he can summon lots of other demons.
The next wave are, among others, teleporting demons and Whiplashes. Ice bombs and self-propelled rocket launchers are the best choice to smash these enemies. A few Mancubus demons will arrive at the end. After winning the battle you will receive the last Empyrean key and finally collect your reward.