Riaru de Reberu Age Shitara Hobo Chītona Jinsei ni Natta - Chapter 339: Viper and Mongoose

The Return of the Condor Heroes by Jin Yong was first serialized in newspapers from between 20 May 1959 and 5 July 1961.

This is the second book of the Condor Trilogy. The story has undergone two revisions. The second edition was released in May 1976 and and the third edition was published in January 2003.

An ineffable rebirth, taking advantage of a mystical magic cube that was produced from rebirthing to pass through various worlds(anime, novel, movie, games, etc), collecting whatever I want to obtain!

- Description from MTLNovel

Su Xiao is a cold-blooded young adult who’s seeking vengeance for the death of his family, yet things ended up badly for him.

As he was taking his last breaths, he heard a voice calling him from dimensions, it was a strange system reaching him, asking him if he wanted to be saved, to be Reincarnated.

He just needed to accept a contract, Su Xiao didn’t have any choice but to do so, but he didn’t know what he signed up for!!

Travelling through worlds, doing extremely dangerous missions, collecting special items, levelling up, and even killing Protagonists…

But Su Xiao wasn’t any ordinary Contractor, Su Xiao was a Hunter!

The MC crosses over to: One Piece, Tokyo Ghoul, Naruto, Fullmetal Alchemist, Attack on Titans… and Author’s Original Worlds…

He is a young man without a spirit root. It is believed that this denies him the chance at cultivation. However, a mysterious lady imparts a Supreme Demon Classic to him and from then on, he starts his path of cultivation.
He undergoes a total transformation to emerge as the most fearsome and powerful fiendish demon in his era, that even immortals and fiends are fearful of him, and saints are at his beck and call.
Mastering the Supreme Demon Classic is the turning point in his life. He is able to unleash his immense and divine power from within, illuminating the entire universe!

- Description from Novelupdates