MTL - Naruto Lineage Theory? Trash - Chapter 684

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

I opened my eyes to find myself in another world. For some odd reason, I ended up as a sword. Before my eyes laid a plain full of magic beasts, and so, I launched my body and flew in search of a partner, a wielder (females only).

In a world where the boundaries between dimensions are thinning, a young man named Kael Ardent is thrust into the Eternal Nexus, a realm where countless worlds converge. Here, the laws of reality are dictated by Aether, a mystical energy that fuels both creation and destruction. Kael discovers he is a Nexus Warden, a rare being capable of manipulating Aether to bridge worlds and command the forces of the Nexus.But the Nexus is not a place of peace. It is a battleground for Eldari, ancient beings who seek to control the Nexus and use its power to dominate all existence. Kael must navigate this treacherous realm, forge alliances with warriors from other worlds, and unlock the secrets of his own latent powers. As he grows stronger, he uncovers a prophecy that foretells the rise of a Prime Warden, a hero who will either save the Nexus or doom it to eternal chaos.

Humans, once the masters of Earth, were losing their place to the inexplicable phenomena known as Objects.And this is a story about becoming an Object and living worry-free in the Seoul of such a world.