Aria T'Loak has requested assistance uniting mercenary bands under her control. Speak to Commander Bailey's assistant at the Presidium Commons C-Sec station and gain the allegiance of the Eclipse band.
Walkthrough Editfounder Jona Sederis. Talk to Commander Bailey at the Citadel Embassies who will insist that you talk to her before he will release her. Head down to the commons and over to the C-Sec outpost to talk with her. After that you can speak to Bailey to secure her release, or you can track down the salarian Sayn, Jona's second-in-command who leads the group in her absence, to take over in her place. Sayn is on the Docks: Holding Area level of the Citadel, in the processing area near Garrus and the Shepard VI. You can convince him to leave Sederis to rot in jail or to kill her upon her release. Either way, Sayn will take over the Eclipse.
If you decided to not free Sederis and convinced her second-in-command to assume leadership of the Eclipse, there is a bug. After talking to Sayn, if you immediately save and exit the game, upon returning to the game the necessary conversation with C-Sec will not trigger and renders this mission and Aria's tri-mission arc unable to complete. A suggested workaround is that after talking to Sayn, head to the elevator immediately. The conversation with Bailey triggers once you pass through the scanner. When that happens, the bug is averted.
Alternatively, a forum thread has provided details for a workaround.