MTL - Nine Realms Sword Master - Chapter 25 Qianlong Ring

Zheng Tan somehow returns to the year 2003, turning into a black cat. Picked up from a rubbish heap by the Jiao family, he is given the name “Charcoal”, and so begins the life inside Professor Jiao’s family. Zheng Tan remembers that he was once a person, but currently his body is that of a cat. He has no way of speaking the human language, but can understand it, and even retains the thinking and IQ of a young 20 year old youth. Read along as he adapts to this new world, and somehow tumbles from a group brawl to catching a thief.

- Description from Novelupdates

In a world ruled by strength and chaos, a soul from modern Earth awakens in the frail body of a seven-year-old Sakazuki. Orphaned by pirates and left with nothing but a burning hatred, he refuses to remain weak. With knowledge from his past life and the power of the Magma Magma Fruit, he begins his ruthless ascent through the ranks of the Marines.But justice is not so simple. Corruption festers within the very institution meant to uphold order, and the line between protector and tyrant blurs with every battle. Undeterred, Sakazuki will forge his own path—one paved with fire and blood.From a vengeful child to the most feared force in the Marines, witness the rise of a man who will set the seas ablaze and redefine what it means to bring order to a chaotic world.

5 years after the world changed, the final boss appeared.