As part of the story, you’ll have to rescue Gareth from Braccus’s Armory. Later on, after you escape Fort Joy, you’ll arrive at the Seekers camp. However, you can only remove the collar on your player character. Complete the trial, and you’ll be able to talk to the tinker (located back in Fort Joy, not the underground area) to remove the Source collar. This will take you down to an underground passage where you can attempt a combat trial. When you first arrive at Fort Joy, examine the hatch on the ground near Griff.

However, there is a way to remove them, though you’ll need to be a little eagle-eyed. This prevents you from using any Source powers or magic, essentially putting you at a slight disadvantage when you’re in combat. When you first start out in Divinity: Original Sin 2, your character, along with all potential party members, will have a Source collar strapped to your neck. Removing All Source Collars in Divinity: Original Sin 2