Deadites are one of the monsters seen on the whiteboard. They were bet on by the Story Department.
In the Evil Dead, Deadites are people who have become possessed by a demonic or evil spirit, transforming them into an undead monstrosity. The anatomy of a possessed person alters slightly after becoming possessed; the teeth grow sharper, eyes turn pale, and skin decays slightly. 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.
The Deadites are a direct reference to the Deadites of the Evil Dead series.