How come this arcanist is invincible?

"What's the greatest advantage of an arcanist?" "Creating their own spells, not relying on the magic network, a high ceiling, and closer to the essence of magic." "And the disadvantages?" "A fragile body, slow growth, difficulty reaching the later stages, and extreme vulnerability to close combat from warriors and assassins." "What if... an arcanist possessed immortality?" "...Huh?" After transmigrating, Kane, chosen as a sacrifice, awakened his immortality in a desperate situation. Thus began the journey of an immortal grand arcanist to godhood! [Pseudo-D&D, mixed worldview, system, wish-fulfillment story]