Quest from: Lord Nasher;
You'll meet the new Luskan ambassador, Syney Natale (funny because you know her from Qara-related cut-scenes and already can tell that she's up to no good). After a short speech you'll know that your next target will be the Crossroads Keep, a fortress that now serves as Black Garius' base of operations.
Before the walls you'll meet a unit of Neverwinter knights led by Vale. When he gives you the sign, join his men in battle. Defeat all the enemies near the walls and on the courtyard. Apparently the mages can't deal with the magically sealed gate, so you'll have to try the other way - a secret escape tunnel. You'll find the entrance on your world map.
In the tunnel you'll be fighting a few kinds of overgrown beetles. Notice the ore deposit ($) and make sure you get experience for finding it - you'll make good use of ore soon enough.
When you reach the keep, you'll find yourself in Aldanon's cell (1). Talk to the elder and look through the bookshelves around to grab the Ancient Art of Golemcraft. In the next room you'll have to fight a large group of enemies. Eventually Vale will appear and support you. The key that you need to proceed is by one of the dead mages' body (2). You now have to face some shadow priests (3). When you defeat them, head down.
A blade golem will attack you as soon as you enter. Defeat it and go further. You'll now have to kill Black Garius's bodyguards that secure the ritual. Their leader will be consumed by evil magic and drop dead without you raising your fist. Take the shard from his body and talk to Vale to go back to Neverwinter.