Oblivion - Main Quest 5 - Weynon Priory

After closing the Oblivion Gate in Kvatch, you are to take Martin to Weynon Priory. If you haven't brought Martin with you, Jauffre will ride to wherever Martin is located after finding out the Amulet is missing. Upon arrival, Eronor will run to greet you with some bad news: Mythic Dawn agents are attacking the priory. If you attack the Agents before Eronor has a chance to speak to you, it will count as an assault - this is difficult to do, however, since Eronor is already outside, waiting for you to arrive and is scripted to come to you even if you are invisible. In the name of the late Prior Maborel, you must kill these assassins with the help of Brother Piner. After they're dead, you'll find that Jauffre has his hands full killing more agents in the chapel. Head over there and help him finish them off.
After you help Jauffre kill off the assassins, he will want to check on the Amulet of Kings. As he suspected, the amulet has been taken and will need to be recovered. Jauffre suggests taking Martin to Cloud Ruler Temple, a safe haven for the Blades located to the northwest of Bruma. When you arrive, follow Martin as he is welcomed and hailed by the Blades as Emperor Martin Septim, Dragon Born. After Martin gives a speech to the soldiers, he speaks with you about the importance of recovering the Amulet of Kings.
Jauffre will offer you membership into the Blades. If you accept you will be given an Akaviri Katana and you will have access to all the beds, books, armor, and weapons inside the temple. After you make a decision about membership, Jauffre will give you your next quest. It is not necessary to join the Blades in order to continue with the main quest, nor is this your only chance. If you later change your mind, you can just talk to Jauffre again and choose the topic "Join the Blades" — as long as Jauffre is still alive (he can die during The Defense of Bruma or Light the Dragonfires, at which point it becomes impossible to join the Blades) and his dispositiontowards you is 40 or higher. Cloud Ruler Temple is not the only place made available if you join the Blades. As a chapter-house of the Order of Talos, you will be granted free access to Weynon Priory if you join.