To get the quest, talk to Aria TLoak in the Purgatory Bar.
Make sure you check your emails first.
When to start
After Priority: Palaven, and before Priority: Surkesk
Aria TLoak has requested assistance uniting mercenary bands under her control.
Speak to Narl in the Presidium Commons and gain the allegiance of the Blood Pack.
Terminus Fleet War Asset
To complete Aria: Blood Pack, go to the Presidium Commons and locate Narl (point 2).
Watch the cutscene that starts when you enter the apartment, and play his little game, if you wish to earn additional Paragon Points.
After the cutscene you will get several rewards, including the Terminus Fleet War Asset, and credits.