Immortal In The Magic World novel 

Eli was transmigrated to a world of magic and became immortal.
He had endless life and could never die.
“Huh? I’m talentless? Then, I’ll just train for as long as I need, even if it takes 1000 years. That should be enough for me to become strong.”
“The skill is hard to learn? Then, I’ll take a few dozens of years to research it. There’s no way it’s that hard, right?”
“What? I can sacrifice my life force to gain spirit power? That’s a bargain!”
“The enemy is here? Hell, I’ll just hide for 10000 years. Let’s see if he can outlive me. I’ll come out once he’s dead.”
This is the story of an immortal who first struggled to live in the world of magic until he became strong enough to slaughter endless worlds.

Immortal In The Magic World