Endride

The story is set on Endra, an unknown world beneath the surface of Earth itself. It is another world, where the giant Adamas shines above and seven-colored bands drift across the skies. One day, Shun Asanaga — an ordinary yet positive and bright 15-year-old middle school boy who lived in the "Land Above" — lands in this place. He had discovered a mysterious crystal at his scientific businessman father's company, and the moment he touched the crystal, he was transported to Endra. He meets Prince Emirio, who strongly despises the current King Deruzain and had just turned 16 at Endra's castle. He became of age to ascend to the throne, and he wields a weapon to seek his vengeance at last, but the difference in strength proves to be insurmountable. Captured and imprisoned by Deruzain, Emirio is heartbroken. But then, the walls of the prison suddenly become distorted, and Shun appears within them. Two youths live in Endra — one to return his former world, and one to exact revenge. The future that is within the grasp of two youths raised in different worlds… (Source: Anime News Network)

Isekai