The Battle Of The Mind [1]!
Sebastian was first attacked by the Mind Entanglement skill before the Mind Possession skill at the end and he was immediately relegated to a position where he could only suffer this oppression passively.
Though the Sea King was at its weakest state in centuries, it was still stronger than Sebastian by a despairing margin.
It could bully Sebastian with just its soul power alone.
When the attack plan was made by Sage 54 to deal with the Sea King, Sebastian had a lot of misgivings about the plan but he had no choice but to comply. A beggar was not a chooser.
Since the Sea King already had its eyes set on him, not deciding to comply with the Sage's arrangements was plain stupid. It was not like he had a better idea to annihilate an injured Sea King.
It was not every day that he would encounter circumstances to enter a mana-saturated state to battle higher-tiered predators, and this time, even entering a mana-saturated state could not save him.