Rex already knew that the path he walked on would lead him to become a God.
He already swore that he'd destroy the pantheon of Lunirich Gods, and doing so would lead him to be the replacement for them. It doesn't come as a surprise to him when there is another Scion out there, who has the sole drive to become a God like Arthur did.
Looking at Arthur, who reached for the sky, Rex's expression tensed.
Back then, when I begged to survive and the System never appeared, I wonder what I would feel?
Just seeing Arthur's broken expression alone made it clear that the feeling must be devastating.
Being ignored like this must be the worst feeling someone could experience.
"Arthur awakened the Light and Divine elements, and climbed the rank of power rapidly like you did," Kei Xun continued, swiping her hand as the surroundings blurred—and changed again. "He beat all his enemies, saved his village, and founded the Last Forsaken Sect."