Aria will tell you to meet with her contact, Narl, on the Presidium Commons. Go to Apartment 86 at the Presidium Commons. Narl explains to Shepard that he told the Blood Pack he's turning Shepard over to Kreete, a Blood Pack leader, but Narl also explains that this is a lie. The real goal is to lure the Blood Pack out since Kreete is planning to betray Aria. But Narl needs Shepard to play along and pretend that the Commander is captured. Shepard can either respond positively or negatively to this request, but ends up going along with it regardless.
Then three vorcha arrive, one of whom is Kreete. As they are about to seal the deal, Narl reveals that the deal is between Aria and Gryll, to Kreete's surprise. Gryll, who's next-in-line to assume leadership, orders the kill on Kreete. After the kill, Shepard advises Gryll not to double-cross Aria, to which Gryll agrees and admits he's ambitious, but not mad.