Knights of the Thorn

From the OblivioWiki – your guide to everything Oblivion

The Knights of the Thorn is a minor joinable faction that is part of the main quest. As part of the optional quest Allies for Bruma, you will need to close the Oblivion Gate near the major cities. The gate near Cheydinhal will have at least 4 guards surrounding it. Talking to the Count will begin the quest The Wayward Knight and that the Count's son along with his Knights of the Thorn went in but still haven't returned. You will later meet him in there, and the three of you will fight to take the Sigil Stone. If he lives, after you destroy the gate talk to him and he makes you an honorary member of the Knights. If he doesn't live talk to the count again and you will become an Honorary Knight.

In the Shivering Isles expansion in Hale, a former member of the Knights of the Thorn, Pyke, asks you to retrieve his Knights of the Thorn amulet from the Grummite lair - Fetid Grove in the side quest, To Help A Hero. If you do, you are rewarded with the Thorn Shield, with leveled fortify block and reflect spell enchantments.