Invincible at the Start

A brief description of the manhwa Invincible at the Start:

As a nerd, Chen Changan travels through the fantasy world, facing countless monsters and ghosts from the outside world. He initially chose to stay in his invincible field—in which he is invincible—so as not to become immortal before it was necessary. When he gets bored, he occasionally raises some cute and sweet apprentices; in the process of raising them, they become leaders in the spiritual world, which turned the world upside down.