You will need to use all six Empyrean Keys to activate the Maykr Device in the Fortress of Doom, which unlocks the Unmayker.
The keys are awarded for completing Slayer Gates in the following locations:
- Mission 2: Exultia – Hell (Precipice of Recompense)
- Mission 3: Cultist Base – Cultist Base (Promenade of Culling)
- Mission 5: Super Gore Nest – Upper Area (Vermilion Canal)
- Mission 6: Arc Complex – Vargo Convention Center
- Mission 7: Mars Core – Meteor (Delta Labs)
- Mission 9: Taras Nabad (Catacombs)