When you enter the sanctuary, Black Garius will welcome you. Before the battle begins, he'll try to deceive some of your companions and make them turn to his side - it's all about Influence checks. Here are the possible outcomes:
- Bishop betrayed you earlier, but if your influence on him is high enough, he'll refuse to fight on Garius' side and just leave. In the other case he'll fight together with your enemy and...
- ... tell you that he can now control the Construct. An influence check on Grobnar will be done, and if it succeeds, the gnome will be able to keep the golem at your side.
- Garius will try to tempt Neeshka. She can either join him or stay with you.
- The reavers' leader will then try to convince Sand and Qara to join his side. They hate each other so much that you will always loose one of them and keep the other - obviously, the one with higher influence will stay.
- At the end Garius tries to lure Ammon, but the warlock cannot be actually convinced to betray you, the influence check is only the matter of the response he gives him.
HINT: If you want to make things easier in the following battle, after you see the outcome of influence check revert to the previous save and take all the equipment from NPC that are going to betray you.
You'll be separated from Garius with a magical barrier. A Balor will be summoned behind your back, so take it out in the first place. Some greaters shadows will appear when he goes down, they are no real threat.
The magical barriers seem to be quite buggy and there's no real logic to their behavior. Truth is, you should be able to take some of the characters to the other side and fight the enemies directly. First take down your former companions that decided to abandon you (serves them right). You don't have to recite Garius' true name for the second time, he's vulnerable from the start, so just hit him as hard as you can and he will go down sooner or later.