4 books in series

Seven Princes of the Thousand Years Labyrinth

Seven Princes of the Thousand Years Labyrinth is a 3-book series with 4 primary works first released in 2015 with contributions by Aikawa Yu, Atori Haruno, 花鶏 ハルノ, and 相川 有.

Sennen Meikyu no Nana Oji Seven prince of the thousand years Labyrinth 1
Sennen Meikyu no Nana Oji Seven prince of the thousand years Labyrinth 2
千年迷宮の七王子 Seven prince of the thousand years Labyrinth 3
千年迷宮の七王子 Seven prince of the thousand years Labyrinth 4