I Got a Cheat Skill in Another World and Became Unrivaled in the Real World, Too, Vol. 4 (Manga): Volume 4

While exploring the Weald, Yuuya Tenjou happens upon a mysterious cave. Inside, he finds a document penned by a legendary sage that he uses to inherit their mana pathways, gaining exceptional skills and all-powerful magic. Now there's nothing that can stop Yuuya--not even the laws of nature!