When the player first arrives at the Sanctuary of Amadia speak to the masked priestess near the waypoint shrine, she'll tell the player about how she told Gareth where to find source draining weapons to fight the Magisters and their Shreikers.
She'll guide you to go north to the old Braccus Armoury where you'll find Gareth. This quest intertwines with Most Dangerous When Cornered.
Defeat the Magisters, speak to Gareth and return to the sanctuary.
Speak to Gratiana and she'll tell the player about another vault the Magisters don't know of yet. She'll also ask the player to bring her the Soul Jars located within.
Return to Gratiana after you've acquired the jars and speak to her and make your choices as to what happens.