Gathering Martial Arts to Transcend in a World of Demons and the Weird

[A clever protagonist, invincible, no female lead] In the vast underworld, unchecked growth of demons, monsters, and the uncanny has damaged its very foundations. Forced to remove all the uncanny beings and demons, it wanders through time and space. In the end, it merges with the protagonist, who slays demons and destroys the uncanny to restore the foundations of the vast underworld. The vast underworld grants the protagonist cultivation techniques and secret manuals accumulated over millions of years, along with energy gathered from the demons and ghosts, helping him improve his cultivation. Slay a snake demon and gain the Dragon-Elephant Prajna Skill... Slay a knocking phantom and gain the Heavenly Blade's Nine Questions...