MTL - Marvel: Unlimited Possession - Chapter 615 captivity

An ordinary student accidentally traveled into an other universe and became the king of a small kingdom. He had gotten a “Cheating Code” of life and was able to complete something extraordinary. This is the story about a king that was able to conquer continents and leave behind him an ultimate legend that will be recited between every generation to come.

- Description from novelupdates

[ Fusion Successful ][ Water Toad + Sky Dragonfly + Nightmare Moth + Bluefang Hyena= Nightmare Hydra ]----Yuki Igarashi, once a terminally ill boy confined to a hospital bed, dies with a silent wish for strength and freedom.That wish is mysteriously granted when he reincarnates into a fantastical new world. Not as a human, but as a newborn Frost Dragon.With his memories intact and curiosity alive, he is soon greeted by a mysterious system called the Dragon God System, which mistakenly binds itself to him, claiming he is not its original host. Still, it is now eternally bound to him.Despite his infant form and weak body, Yuki discovers he can extract bloodlines and skills from other creatures using the Dragon God Codex, and fuse them within a mysterious space called the Fusion Lab.Even more shocking, a glitch in the system allows him to repeatedly Fuse bloodlines without depleting them, opening the door to limitless growth.

An ultimate warrior, the one and only Alchemy Emperor, Ling Han, died in his quest to achieve godhood. Ten thousands years later, he, with the ‘Invincible Heaven Scroll’, was reborn in the body of a young man with the same name. From then on, winds and clouds moved as he set himself against the uncountable geniuses of this new era. His path of becoming a legend has begun once more. Of all eras underneath the heavens, I am the strongest!

- Description from Novelupdates