The quest to obtain your third and final soul is given by Cyril Kalmar at Valor Hold. He wants you to deal with a little problem before granting it to you. Orphiel is deploying a corruptor at Ascendant Hall. Cyril wants you to put a stop to it.
Ascendant Hall is Northeast of Valor Hold. Check out your map for the exact location, then head out to take care of business.
The hall is the big scary-looking structure atop the hill to the Northeast. To get there, you’ll have to head up a road to your right, which is littered with Carrion Flayers. Fortunately, you have a quest that involves killing them, so you’ll profit from clearing the path up the hill.
About halfway up, Orphiel himself will appear and advise you to turn back, as nothing can stop him from accomplishing his goal. You can attack him here, so just laugh and keep moving up the hill.
Orphiel’s corruptor is currently powering up Ethect, his construct. Powering down the machine will cause Ethect to attack, so be ready for a fight.
After Ethect is down, Orphiel will appear and attack, as well. He’s a bit of a coward, though, and after you’ve whittled his health down a bit, he’ll stun you and run away.
Once the stun wears off, you should notice a Messenger of the Vigil standing immediately in front of you, on the precipice overlooking Valor Hold. Run out and speak to him to complete the quest and receive your third and final soul.
Once you’ve chosen your soul, speak to the Messenger again to receive a free teleport back to Valor Hold.
Congrats, you’ve completed all three Guardian soul quests and are now a fully powered-up ascended. How you choose to spend your talent points is up to you. Now get out there and continue to battle the forces of Regulos.