- Speak with Brynjolf and Karliah at the Thieves’ Guild.
- Meet Karliah at the standing stone just outside Riften.
- Enter the Nightingales’ Cave and follow Karliah’s instructions.
- Equip the Nightingale Armor.
- Proceed further into the cave and follow Karliah’s instructions.
- Speak to Brynjolf about the leadership of the Guild.
Return to the Thieves’ Guild and show Brynjolf what you found at Riftweald Manor. He will explain the significance of your findings, and tell you to speak to Karliah.
When you speak to Karliah, she makes a reference to being at full strength before pursuing Mercer, and asks you and Brynjolf to meet her at the standing stone outside of Riften. Nearby is the entrance to the Nightingales’ Cave. Proceed inside, but don’t get too far ahead – Karliah tells you more about the Nightingales and their role.
Your first stop is the armor chamber. Activate the stone; the Nightingale Armor is placed in your inventory. You must don it before continuing. Follow Karliah further into the cave; side passages lead to sleeping quarters and living spaces. She will later remark that this was the Nightingales’ base, and that she will return here once done with Mercer. Karliah is the only one who can operate the pull chain for the gate leading into the next area.
You are directed to stand on a specific platform. If you have a companion with you, their presence does not affect the outcome in any way. Karliah summons Nocturnal and asks to resume her role as a Nightingale as well as induct you and Brynjolf into their ranks. Nocturnal agrees. If asked, Karliah will explain the role of the Nightingales, and also how Mercer got into the vault without having the necessary keys. Brynjolf will address the issue of the leadership of the Thieves’ Guild, saying that he and Karliah think you would be the right person for the job.
But you still have to find Mercer and stop him.