The heavenly return system appears, I choose to become a living sword.

Li Shen lets himself be devoured by his demonic sword, seeing it as his only option to achieve immortality. After more than nine hundred years, his system finally activates after his soul is devoured, and he awakens the Heavenly return system, allowing him to return to when he first transmitigated as many times as it takes, and even keep some of his hard-earned achievements. Only due to a system error, he becomes more than human, returning with two demonic souls, and the ability to cultivate like a demonic sword, directly harvesting the qi of those he's killed. Note: this is not a story starring a sword, instead the main character cultivates his body both as a human and as a sword. Image: Not mine originally, though I made some edits, if it's yours and you want it taken down let me know.