My first task from Hollow City is to refill it by saving as many people as I can from the various torturous dremora in Coldharbour, most of whom are Fighter and Mages guild members. There are others here too whom I ally with, such as elf-eating Lamias, Argonians who have sold the soul of their Hist tree to Molag Bal, and a repentant Angoff (the druid terrorist leader from before).
Once the force is assembled it's simply a matter of pushing forward and upward in Coldharbour, slaying the armies of vampires, dremora, and daedra in the way. Molag Bal shows up in person to try stop us but as expected Meridia reveals herself to duel with him while the rest of us continue with the mission of destroying the Planemeld - aka the doohickey that Molag Bal requires for his grand plan.
Whatever you say, boss!
Many / most of my allies die in the experience, but we are ultimately victorious in "planting the bombs and blowing things up" which pretty much puts a stop on Molag Bal's world-killing plan. Meridia takes us survivors back to the Hollow City for celebrations. Wish they made it more clear who made it and who didn't though. What is this, the Deathly Hallows or something?