Dragon Raja III: Back Moon Tide

When a mysterious heartbeat is detected under the seas of Japan, Luminous, Caesar Gattuso and Johann Chu, (also known as the Ace Team of Cassell College) get sent on a mission to Tokyo, where they must cooperate with Cassell's Japanese branch, led by the conflicted Chisei Gen.
After an unexpected turn of events, the trio will find itself in the middle of a hybrid mafia war.

Adapted from the original novel which has also been turned into a game.