Soul O'Brien serves as a foil to Marcus, showing that those who claim to be "good" can become the very monsters they hunt.
Nathan finally reaches his 17th birthday. According to prophecy and witch law, he must consume the blood of an ancestor (his father, Marcus) to receive his "Giving" and unlock his full powers. The Bastard Son & the Devil Himself s01e08
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. Soul O'Brien serves as a foil to Marcus,
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 Nathan finds Marcus Edge
The episode reinforces that Nathan is not "born evil" because of his father; his choices define him.
Nathan, now empowered after his Giving, engages Soul. The battle is a clash of Nathan's raw, emerging abilities against Soul's stolen, versatile powers.