In the Evil Dead, Deadites are the bodies of human beings who have been possessed and then animated by demonic energies or by evil spirits, prolonged possession of which transforming them into an undead monstrosity, although the can take momentary normal human forms. The anatomy of the possessed person alters slightly after becoming possessed; the teeth grow sharper, eyes turn pale, and skin decays slightly, possibly symptoms of the possession and deterioration of the body the entity is animating. The longer someone remains possessed by a demonic spirit, the more hideous they are in appearance. It is never revealed whether the Deadites in The Cabin In The Woods function in this way as well.
A tape recording with verses from the Necronomicon summons them just as it did in the original Evil Dead and hinted towards during the end credits of the 2013 Evil Dead remake.
It is possible the Deadites were a hint towards the 2013 remake/reboot/continuation of Evil Dead.
The Deadites are a direct reference to the Deadites of the Evil Dead series.