From the Kalkys Firing Platform's lift you will be immediately ambushed by the usual suspects. Once you've dealt with them, follow the objective marker to the console on your right-hand side and press // to close it. Objective Complete!
Titus will call Inquisitor Drogan to let him know that the damage has been repaired. You need to proceed to the main control panel, with a new objective in hand: Fire the Psychic Scourge. Head upstairs and hit the switch on your left. Congratulations, you've fired the Scourge.
But what's this? Warboss Grimskull's ship comes out of nowhere. Surely it's boss fight time, right? ...Nope.
Watch the cutscene. It turns that the Psychic Scourge didn't work, and instead of getting rid of the Orks, it allowed Daemons to warp in and start killing humans and Orks alike. Also Drogan is now working for Lord Nemeroth and the Daemons. And then Drogan dies, but it turns out that he's actually been dead all along. And on top of all that, you don't even get to have a proper boss fight with Grimskull, because he gets killed trying to fight Nemeroth.
Now that you're finally back in control of Titus, it's time to get out as fast as you can. While you're running out, you'll be accosted by Bloodletter Daemons. These guys are most vulnerable to melee attacks, and will teleport away if shot, so move in on them and wail away. Keep running toward the lift and get inside. Once you've descended the lift, more Daemons will attack. Your squadmates can shoot them without fear of teleportation, so go up to the Daemons engaged with your AI buddies and attack them first. Run to the objective marker and push open the door.
Once the door opens, you'll see a massive fight between a group of Orks and Bloodletter Daemons. Jump in and slaughter everyone. Once everyone's dead, hit the switch on the far end of the map to escape the facility, ending the chapter for good.