User blog comment:Crimson and Darkness/Volume 24 discussion blog/@comment-39.43.169.255-20180103015838/@comment-29963364-20180103031503

The "Heavenly Kings," in this case, is a reference to the Toriko series.

The main character, Toriko, is one of four Gourmet Hunters known as the "Heavenly Kings."

Gourmet Hunter is basically an adventurer/explorer who travels the world looking for animals & plants (ingredients) that can be eaten.

The "Heavenly Kings" are known for discovering & hunting the rarest, most dangerous, and most delicious ingredients ever.

So Issei calling his mother, Rias, Akeno and Asia the "Heavenly Kings" is a reference to their cooking ability.