He could clearly remember. The first place that Hellgate went after his awakening was the Ancestral God Grounds.
By obtaining Cthulhu’s Origin, he had already achieved his purpose; here was no point in remaining here. Next, he had to find the previous Thunder God, Hannet, to filter Cthulhu’s will. If not, he couldn’t use the Cthulhu’s Origin on hand.
Having the advantage as the King of the Century, once Garen entered fifth form, his battle ability would instantly hit its peak; on par with Phiroth and the Cthulhu King. He had already trained his martial arts to a complete level, totally different from the rest of the fifth forms who had to start their martial arts training from scratch. This was his advantage.
