You can complete this quest in two ways. The first one is talking to one of the four Seekers staying at the Sanctuary Of Amdia . You learn that their leader - Gareth - has gone to find a weapon to get out of Fort Joy and he did not return. You can offer your help.
1. Sanctuary Of Amadia - Samadel, Kerban, Gratiana, Exter
2. Old Ruins - Gareth
You find Gareth in the Old Ruins , where he is fighting a group of magisters. If you did not talk to his companions before, the quest begins upon discovering the location. It is important to take the path on the wall and avoid the Shrieker. After you get inside you will start talking to the magisters. If you have at least one companion with one or more persuasion points you can convince the magisters to leave after you purport to be an undercover magister and mention Dallis' new orders. Then, go deeper inside the ruins, where Gareth is fighting another group of magisters. There are three endings you can choose.
Ending 1: Gareth lives
If you decide to join Gareth and you manage to save him, he will talk to you. He will tell you he can help with removing the collars. Moreover, he will mention the plan of escaping Fort Joy using the magisters' boat. First, you have to acquire the weapon allowing you the kill Alexander. The Call To Arms quest is unlocked.
Reward: 3 guaranteed rewards and 4 to choose from depending on classes in your team
Ending 2: Gareth dies
You can side with the magisters. As you outnumber Gareth, there will be no problem with killing him. Gareth's death blocks the option to complete the Call To Arms quest.
Reward: 2240 experience points
Ending 3: No action has been taken
You can also leave Gareth alone. Similarly to the second option, Gareth dies and you are unable to complete the Call To Arms quest.