When you're back to the Keep, you'll have to make some decisions concerning the forthcoming battle. Most importantly, you are to choose two companions that will fight by your side during the siege. It's good to go with Qara, as her fire-based magic will prove essential to destroying the siege towers. You can also choose the supporting units that will be on the battlefields with you - the range of possibilities depends on which alliances you made before. The battle itself is not easy at all, but in the first phase you'll only be facing regular undead troops, no bosses nor shadow reavers. Destroying the siege towers will save you a lot of trouble, so focus on them first. When you hold off all of the towers that can be seen from the distance, you'll be taken to the courtyard.
Bishop will betray you and tell you that he purposely destroyed the gate mechanism so that you can't close it. Well, his loss, really. You can now change you buddies, choosing 2 other characters to support you. When you defeat enough enemies, Black Garius will show himself and make a little mess, summoning the Shadowalker (Shadow King's avatar). Focus your attacks on the shadow beast, avoiding Garius' magic whenever possible. You must have the Sword of Gith equipped to deal real damage to it. When the creature is down, choose either Zhjaeve or Ammon to recite Garius' true name - after a while the enemy leader will have to retreat. Hazzaa!