Wicked Dragon (邪竜 Jaryū), also known as Elder Dragon (エルダードラゴン Erudā Doragon), is a tribe related to the Dingir.
They are originally sealed in the Dynamis before being unleashed to the surface world by Gilgamesh. Defeating those dragons are the main story plot during Heroic Saga X.
Z/X Encyclopedia Entry
Also referred to as Elder Dragon, they are dragons that dwells in the Dynamis. Their origins are unknown. Because they have excessively received the Dingir's influence, despite having reached Depth IV—which by all rights is not surprising if their soul incinerates anytime, they are unique existences that has not yet perished.
Bahamut, Leviathan, Hraesvelg, Fafnir, and Nidhogg are the five confirmed Wicked Dragons.
All Wicked Dragons are Cost 8 Z/X with Power 12000 and Level Ability that triggered when they appear in Square. All of them were released during the course of Heroic Saga X event.