From Lexicanum

A Hellion is a Wych who is mounted on a "skyboard", a flying platform from which they assault the enemy forces before they have a chance to fire back. They wield Hellglaives, double sided spears with small splinter rifles built in to ensure maximum damage to the enemy. Hellions perform mad races in the shady streets and between the spires of Commorragh as well as in the arenas. They often deploy ahead of the Dark Eldar army, or even during the battle.

Sources

• Codex: Dark Eldar

Dark Eldar Army List
HQ	Archite • Archon • Dracite • Dracon • Haemonculi • Incubus
Elites	Grotesque • Mandrake • Succubus • Warp Beast Pack • Wych
Troops	Raider • Sybarite • Warrior
Fast Attack	Hellion • Reaver Jetbike
Heavy Support	Ravager • Scourge • Talos
Special Characters	Asdrubael Vect • Decapitator • Drazhar • Kruellagh the Vile • Lelith Hesperax • Urien Rakarth