Phibrizzo didn't know when someone casts the Giga Slave spell, he/she calls on the Lord of Nightmares himself. He wouldn't of not proceeded with this plan of more power, if he had known about this.
Lina (Who is actually possessed by the Lord of Nightmares-- note: I'm still refering to him as Lina) gives Phibrizzo what he wants, and kills him.
Everyone (including Gourry, but minus the already dead people) awakes from their humble sleep, and sees Lina. They are shocked to know that Lina is no more, since she is possessed by the Lord of Nightmares.
They place starts to crumble and Lina proceeds to go back to the dark where she came from. Gourry doesn't want to lose Lina, and goes after her. Amelia, Zelgadis, and Sylphiel tries to stop him but it's no use, when more of the debris stop them. Meanwhile with Martina and Zangulus, he suggest to pray to Zoamelgustar for there safety.
As Lina starts to head to the dark ball where she came from, Gourry still goes after her. He goes through