However, Karliah warns the Dragonborn that, since Mercer was a Nightingale, he is very dangerous. Upon meeting with Brynjolf and Karliah in the center of The Ragged Flagon - Cistern, a discussion ensues about executing Mercer for his betrayal to the Guild.
Karliah has informed Brynjolf and I that we're to join the ranks of the Nightingales and assist in reforming the trinity that was disrupted over twenty-five years ago when Mercer Frey murdered Gallus.