The Prince of Demons
The party found themselves in a big oval room and Aerie sensed a horribly evil thing in there. And it was able to convey telepathic messages to Leslie. He understood that he was being decived and decided to check it out. They left the big chamber to confront Odren and his followers about it.
Odren admitted his lies but presumed that Demogorgon was dead and ran inside the Keep.
Then a true follower of Vigil rose up and suggested to seal the Demogorgon and Odren inside. Leslie had more fighting ideas and his party agreed that Demogorgon had to die - here and now.
They went in again, summoned their helpers and Aerie buffed the party with Prot. from evil, Chaotic Commands and other useful spells.
Demogorgon had already killed the Vigil followers and when the party showed up he summoned a bunch of demons to help him. Leslie used Improved alacrity to quickly cast spells and hit Demogorgon with a couple of Pierce Magic and then some ADHW's while softening the demons around him. Jan improve hasted Valygar, Minsc and Keldorn who were all under the effects of Hardiness to be able to withstand many hits from foes. Aerie casted one Storm of Vengeance and then an occasional Breach or Holy Smite to help.
Demogorgon was a real heavy weight - he was only Barely Injured even after many rounds of fighting. Leslie then instructed the guys to concentrate on him only while he and the mage support group fight with demons. He summoned one more mordy and kept attacking the demons with ADHW's and other damaging spells. And Demogorgon was slowly getting weaker and weaker. He summoned one more wave of demons and then the guys finally destroyed him! After his destruction the rest of the fight ended very quickly. The party got a lot of XP and they left the bloody Keep to continue their quest.