Isekai Maou to Shoukan Shoujo Dorei Majutsu - Volume 3 Epilogue

In a world where human and monsters relied on mystical fruits and their inner power to fight for dominance, a lost soul was thrown, given the powers from all the other worlds he’s known. But there was a catch; he had no control over what power he would get! With knowledge beyond that of most, he sails his path to the top of the one piece world!

- Description from Novelupdates

In a game-like world where dungeons, monsters, and players appear.

10 years searching for his lost sibling.

[ 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.