Eight Great Dragon Kings (八大龍王 Hachidairyūō) is a group of prestigious Hourai families in the Green World.
Z/X Encyclopedia Entry
The general name for the prestigious Hourai families that worship the sacred tree called Mougi. As proof of one being a part of the "Eight Great Dragon Kings" that guard Mougi, they carry weapons made from the branches of Mougi.
Since Mougi has long since vanished in the Green World, the families eagerly desire to capture the "holy land" Takachiho of the present, where the sapling of Mougi exists.