"Alright, Guardian. It's time to deal with these fools head on. Morte has scouted the area you surveyed earlier, and determined the whereabouts of the Resistance. Several factions of Avalon have banded together, in the area the Assassins used to claim as their own. If you know what's good for you, you'll go there, and kill them all. To gain access, you'll need to speak to the Gate Keeper, and tell him the password. It's 'Innocence.' Go to the Assassins Guild lair, behind the Crypt."
This is your last quest given to you by Xanthar. Exit the throne room and head down to Lower Avalon. Follow the old path to the Crypt, but pass it by either walking around it on the right or walking up the hill on the left. Locate the giant skull structure and walk up to the Gate Keeper. Right click him, and when he asks for the password, look at him and type "Innocence" into the chat. When he teleports you inside, walk down the steps. The quest will end when you are fully inside. Rather than kill everyone, speak with Cain for the next quest and to join the Resistance.