Fighting is essential in Doom: Eternal. It’s the core of the game’s quality, and engaging in more combat is never a bad thing. Scattered throughout several of the levels are secret encounters you can engage in to receive a higher score, and grab some additional collectibles. There are two on the Cultist Base level.
The one is pretty straightforward to find. You have access to it after you fight your way through the pit, while on the way to retrieve your Super Shotgun. You want to fight through the enemies in the pit, and then make it back to the top. After that, you return to the room where you started, and you need to cross an electrical blood pit to raise lower the gates. After the gates go up, off to the left side is the first secret encounter. You will fight the horde of demons in that enclosed room.
The second and final secret encounter is near the end of the mission. To find it, you need to scale the inside of a cave where there are two pillars with walls you can grab. You need to climb to the top, reach it, and then stop at the point where spiked walls are smashing into each other. To the left side, there’s a room with an armor piece inside of it, and two bombs. The secret encounter is behind the breakable ice wall. You may want to keep the bombs active before you do this encounter, to increase the damage you can do to the demons that spawn here.
Several missions in Doom: Eternal have secret encounters in them. You need to find all of the encounters to complete the Demonic Corruption bar. You can return to a level using mission select to relocate any of the encounters you did not find. The encounters are not repeatable if you have already completed them.