Shrike Abyssal: Prothean Obelisk side mission can be completed after you finish Priority: Palaven.
In the War Room, open the Galaxy Map and enter the Shrike Abyssal system located to the NW.
When to start
After Priority: Palaven
A Volus diplomat needs a Prothean obelisk.
Recover it from the Shrike Abyssal and deliver it to him at the Citadel embassy.
To complete this mission, enter the Shrike Abyssal system and travel to Urla Rast.
Locate Talis Fia and scan it, then enter the orbit.
Start the scanner and hold LT.
Move the scanner, following the white line that appears on it, and when you reach the white dot on the map launch the probe to retrieve the Prothean Obelisk.
Return to the Embassy and when you exit the elevator go left and look for the Volus Diplomat in the waiting room.
Give him the Prothean Obelisk for 20, 000 credits and reputation points.