The Thundro Curse (volume 1) - Manga

The adventure begins when a young boy, Danton Asuhara, is defeated in battle at the hands of a feared dark spirit who is known across the worlds as Inazuma. After dealing a final deadly blow both Danton and Inazuma are suddenly transported to another world. Four years later a portal mysteriously opens between the two worlds drawing the two lost souls back. However, things have changed and new mysteries are formed. What creature is strong enough to open a portal that rips between the two dimensions? What threat does this new entity pose to them in the future?