Achieving the rank of Silencer requires you to purify the Cheydinhal Sanctuary. Ocheeva gives this assignment to you via a letter from Lucien Lachance. The letter instructs you to meet him in Fort Farragut. Rather than entering through the main gate and fighting through the undead guards, look for the secret entrance in the hollow tree immediately to the northwest of the main tower. Lucien orders you to kill all the members of the Cheydinhal Sanctuary. Return to the Sanctuary and eliminate all the other assassins in whatever manner you choose. Try to avoid killing anyone in the presence of the Dark Guardian. Once everyone has been killed, return to Fort Farragut and speak to Lucien Lachance. The achievement will unlock as soon as the initial part of the conversation is complete.