Fushi-Ou wa Slow Life o Kibou Shimasu

After being reincarnated, the protagonist Sigmund awoke in another world as a meager ghost. Fleeing from monsters into a nearby cavern and slowly leveling inside of it, Sigmund had evolved into the all-powerful "Immortal King" before realizing it! Now, using the nearby vast forest as a base, gaining followers, constructing golems and working on building a self-sustained life is Sigmund's new goal. However, there's always trouble lurking around every corner...

(Source: Alpha Manga)