Return to Reaver. You will learn, that he has told Lucien about you. Things however have complicated, and now Reaver is the pray. He will offer you an escape route through some hidden tunnels, but the lord's army will raid them anyway. A good ranged weapon and shooting skills will come in handy now. Follow Reaver. After dealing with waves of enemies you will come to the surface. You will find yourself on a beach, and meet Garth and Hammer. Great Shard will show up after a while.
It's a very strong enemy. To defeat him, you must firstly eliminate all the guards he spawns, and then shoot in the unprotected, shining upper part of the Shard. Look out for his projectiles. He marks them with red light, when you see it on the ground, you better look out. Run to the hill from which you will be attacking the Shard. In the end Theresa will show up, stop Reaver and encourage him to help in defeating Lucien.
Prize: 10000 renown points, Shard.