Kujira no Souretsu

Japan's assassins are controlled by "Kujira" (Japanese for "Whale")...

Kuroo Kitakami lives an ordinary life, unaware of the common knowledge of the underworld. However, when his friends are killed by a mysterious assassin, Kuroo is saved by the assassin Shagura, and upon learning of a conflict to succeed Kujira, Kuroo vows revenge and dives into the seas of ​​hell...

(Source: Shounen Gahousha, translated)