Aftermath
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Drake always imagined Pandemonium being a merciless and ruthless man, and perhaps he was indeed one, but right now he could have easily finished him off.
Yet the only thing he showed him was respect. Apparently he hated Oberon, so he was so pleased about his death that his view of Drake became better.
And perhaps out of this happiness he felt, he decided to not jump on his back and attack him with lethal force.
Drake had already recovered his strength and energies right now, but in that window, made up of a couple of second, he was exposed.
He had unleashed all his strength to defeat this powerful new soul clone of Oberon, so it was only natural he would grow weaker after that, needing to cool down and all.
His transformation using Orochi's Divine Ability brought this drawback, where after the transformation, he would feel exhausted for several seconds, perhaps even a whole minute.