Already completed most of the Locating the Threat questline? Assuming you have, you can pick up a follow on questline from Warden Ystanes.
The Warden can be found at the following locations:
Warden Ystanes in West Athen
Athen West - 386x460
Warden Ystanes in Newland
Newland City - 400x360
Warden Ystanes in Omni-1 Trade
Omni-1 Trade - 300x225
When you speak with the Warden, take on a killing a prisoner, help can be found here Escaped Prisoners.
After killing the prisoner you had a mission for, head back to the warden and get your chosen coloured soul. At this point, you need to choose the "Before I go... " dialog.
The Warden will have a new dialog for you and you should get a new mission.
Soulbound - Information
So, as it was suggested by Warden Ystanes, it is time to head off to Priest Fontain to see if we can get some more information about the soul crystals being left behind by the Prisoners.
Priest Fontain Location:
City of Home, Broken Shores
Broken Shores - 620x1500
Priest Fontain will be able to tell you about the Soul Stones. Make sure you have the set with you (i.e. go and kill enough prisoners to have your set of 6 soul stones) before you start dialog with him as he will ask you to drop one into the dialog with him so he can tell you about it.
After he has spoken to you for a while, your mission should update
Soulbound - Relics
With this mission in your tab, you now need to combine your six souls in the correct order to make an Awakened Soul Crystal.
On the final combine, your mission will update again.
Soulbound - Crystal Clear
Speak now again with Priest Fontain and tell him you have the Awakened Soul Crystal.
On transferring it to him, he will award you with 10m credits. He will now talk with you to give you more information about the Soul Ripper.