Sir Thedret had entered the fort a few weeks earlier also looking for the shield, but was caught by the mages inside who want the shield for their own use. They took his weapons and armor and locked him up, releasing him only to torture him in the hopes of getting information about the traps ahead. They were unable to get anything from him and were planning to kill him soon when the Hero showed up.
If released from his cell he will share some knowledge about one of the puzzles and head out of the fort. He states that he is too weak to be of any help and will rejoin the Hero when he can.
After the Hero returns to the Priory of the Nine with the Shield, they will find Sir Thedret dressed in the standard Knight of the Nine Cuirass with the matching shield, stating that he will join the Knights of the Nine and fight the new enemy threatening the Nine.
He will join the Knight Commander with the rest of the knights in the battle at Garlas Malatar and will fight through the never ending wave of Aurorans so the Hero can make his way to Umaril and slay him, body and soul.
After the battle, the Hero will re-awaken in the under-croft of the Priory, after speaking with the spirit of Sir Amiel Lannus Thedret will come down to see what the noise is. He will explain that after the battle they found the Hero's body next to Umaril's, unwounded but lifeless, they had assumed that they had killed each other and moved the Commander's body, secretly, as to not draw unwanted attention from their enemies. After talking, he will state that it is by the grace of the Nine that the Hero is alive again and will rush to tell the other knights.
After the battle, he will return to the day-to-day duties of running the priory and will follow the Hero if asked.
The Hero has answered the call and has set forth to find the remaining Relics of the Crusader. At the Priory of the Nine, they have discovered from Sir Henrik that the Shield is located in the Fort Bulwark which is along the Cyrodiil-Black Marsh border, a distant east of the Shrine of Nocturnal.