Betruger taunts that you will never find what you seek - the Soul Cube. Make your way through this Helter-Skelteresque corridor. Jack Gilbert's PDA reveals the code 579 for storage locker 104.
Battle two chainsaw zombies -- the plasma gun works well.
Stay left and you'll come to this room. Heal at the health station on the left. Code 579 unlocks the cabinet on the right where you'll find an ammo belt, rockets and an armor vest.
Backtrack down the hallway to this single door. Ready your BFG-9000 before entering.
A cutscene plays. A scientist emerges from the portal, followed by your next boss, the Hell Knight, who tosses the scientist around like a rag doll. A second Hell Knight soon follows.
If you've been good about conserving your BFG ammo this should be no problem. Hold down the trigger to increase the payload of your shots. If you haven't enough BFG ammo then use rockets and be sure to keep moving and keep your distance.
Even with the BFG you want to be a moving target, as the Hell Knights pack a mighty punch. Once both are defeated, a brief cutscene plays.