From Halopedia, the Halo wiki
Hellions are a variation of pure form that are utilized by the Flood as a mobile hive and siege machine. Vaguely reflecting the majesty of the Gravemind, Hellions are able to incubate and shelter massive numbers of smaller parasites. They are formed in order to overwhelm even the most dedicated and stalwart planetary holdouts. Hellions possess the ability to alter their form in order to attack any defenses attempting to stop the Flood's unyielding spread, rending and refashioning their flesh and bone into a living weapon to assault their foes.