There are three switches here: one by the stairs, and two at the sides of the room. If you pull the two switches at the sides, and then pull the switch by the stairs, mana will appear in front of the two switches. Exactly what type of mana is generated is random. This is for those who don't have enough mana to face Korax! On rare occasions, you may even get a Krater of Might or a Mystic Urn. Be careful, though. If you do this too many times, some Dark Bishops might teleport in to join the switch-pulling fun... :) However, after the second hall opens later, this room will be closed, and you won't be able to come back again for the mana. Anyway, thanks to the many people who told me about this. I don't have much time now, so I wouldn't have figured it out myself.
Another word of advice: don't walk up to him to discuss your differences either: that brownish trail isn't a carpet! :) And he also hates uninvited people treading his hall. He can fire lightning upwards with his arm to control different traps in the hall, such as activating fireball spitting devices on the ceiling, or turning almost all the floor in the hall into lava! Beside these, there are also crushers scattered around the hall.
After you hurt him enough, he teleports away like the coward that he is. And he opens two pairs of really packed rooms on the far east and west sides of the hall. These rooms are jam-packed with countless ettins and centaurs! They'll come swarming on top of you, the ettins first, followed by the centaurs, after you've killed all the ettins. You probably need many Discs of Repulsion to keep them from stampeding you to death as you shoot at them.
Once you finish off these troops, the doors on the north side of the hall opens to reveal another big hall beyond. Korax now teleports between several locations in both halls, and sends more of his troops from another two pairs of jam-packed rooms in the second hall. This time, they are Chaos Serpents (perhaps with Brown Serpents mixed in as well). Watch out as they literally rain fireballs on you! Also, be careful when you're running back and forth attacking the monsters. Korax can turn most of the floors in the two halls into lava, and activate fireball traps.
After you litter the two halls with the carcasses of the Serpents, Korax will begin to summon more monsters. Dark Bishops will appear, as well as invisible Chaos Serpents (that shoot the more damaging green fireballs like Brown Serpents!) and invisible centaurs. These invisible monsters don't leave any carcasses when they die: they simply disappear. When you reach this point, don't waste your ammo on these monsters! Kill just enough of them to keep them from getting in your way, and concentrate on Korax! You should have kept enough Kraters of Might to keep you going (there are also a few scattered around the two halls).
Now, gather yourself together, and step into the portal. After all the excitement in the final battle, you can at last sit back and watch the conclusion.