Why Is the Immortal Child So Different After Becoming a Powerful Figure?

Arrogant, hot-tempered big-brother Demon Lord uke x cold, scheming, powerful immortal seme In the early days, Mu Ban picked up a little immortal child from the battlefield between immortals and demons. The child was obviously a true immortal, and Mu Ban wanted to raise an immortal by demons just to mock the Immortal Realm. Later, he realized the child also helped him fight the neighboring Demon Emperor more efficiently. And later still, Mu Ban stared at the sky, unable to understand why he had ended up pinned underneath, too. Younger seme, equally matched leads, uke's perspective. More or less a raising-the-child story.