Unzoku a true demon
In the first battle against Immortui, Quinn had been placed in a difficult situation. For one, he had just gone against one of the other demon kings. During his fight, he had been affected by the red mist energy when the other demon king had transformed into its demon form.
Because of this, the armour that Quinn had worn had broken. It left him in an extremely difficult situation as he faced Immortui. As for the second battle, he was a lot more prepared. The armour had recovered itself, giving him more power and versatility in his skills.
He had been learning from the Champions who had faced Immortui before, and even Sil had given him what advice he could from the battle he had with Immortui. Despite all of this, the battle in the end was still lost. He even had the strength of all of the blood that was currently being spilled in the red space. It just went to show how formidable a foe Immortui was.