I've beaten the game on nightmare. I only have one master level, Arc Complex. How do I unlock more? I saw people play at least one other, the Cultist Base, but that doesn't appear to me.
To unlock the master levels, first complete the campaign then interact with the master level monitor in the fortress of doom (next to the main computer).
The Cultist Base master level was a bonus awarded to those who preordered the game. I wouldn't be surprised if it becomes available by another means later but for now I don't think you can get it if you didn't preorder.
The Cultist Base master level (and other preorder bonuses including the DOOT! Revenant skin and classic Shotgun skin) can now be unlocked for all players by purchasing the mini add-on Rip and Tear DLC Pack