Enter the room, walk a bit forward, cutscene. After cutscene, Serenity automatically makes her way to the boss (unless you press buttons), and finds that the boss is sleeping.
One thing to do: wake her up; it involves a terminal at the top of the room to seal the exit off (and prevent escape).
After that, trying to leave, then return to Alexis and the boss fight begins.

It's apparently possible to defeat the final boss with just Misery's End, and considering the misery he plans on unleashing on Newerth, I'd say Misery's End really brings an end to that misery

First form with level 1 Misery's End (which deals 5 damage), though it's a bit hard due to how large the area is. (He teleports all over the place.)
Second form with level 2 Misery's End (which deals about 7 damage?), leveled up with the help of those creatures and the shockwaves. Standing at his eye and spamming it results in a lot of damage (reaching numbers like 182 total damage). Level 3 Misery's End would be useless against this boss, though.
Third and fourth forms with level 3 Misery's End (first hit deals 10 damage, with additional damage caused by the "blade storm"). The fourth form especially; the things surrounding it can be hit with the level 3 Misery's End and the boss will take damage as well. And then there are the minions spawned that will also get hit, possibly.

Starting at 80 HP, and, using hearts, ended up at 64 HP. After the boss fight, a save point appears by the throne and escape is required.

However... with all the leftover space, I might add a bonus area. A cheerful-sounding one, perhaps?
(If I do, it will have two themes - one with an intro, the other without. The first would play only on the first time, but listening to it in Servant Quarters DR can be a thing.)