Soul O'Brien serves as a foil to Marcus, showing that those who claim to be "good" can become the very monsters they hunt.
With the help of his friends, Nathan manages to defeat Soul. However, the victory is bittersweet, leaving the group as fugitives from both the Blood and Fairborn societies. Ending Explained The Bastard Son & the Devil Himself s01e08
Soul O’Brien, now fully corrupted and wielding multiple stolen powers, leads a brutal assault on the forest hideout. His goal is to kill Marcus and Nathan to "purify" the witch world, though his actions have become those of a tyrant. Soul O'Brien serves as a foil to Marcus,
Nathan finds Marcus Edge. Unlike the monster described by the Council, Marcus is weary and philosophical. He reveals the truth about his past and offers Nathan his blood, not through a fight, but as a choice. Ending Explained Soul O’Brien, now fully corrupted and
Nathan discovers his gift is "Power Mimicry" (or a variation of his father's ability to take/use others' gifts), making him one of the most powerful witches alive.