  A hidden sector with a mass gap undergoes an epoch of cannibalism if number
changing interactions are active when the temperature drops below the mass of
the lightest hidden particle. During cannibalism, the hidden sector temperature
decreases only logarithmically with the scale factor. We consider the
possibility that dark matter resides in a hidden sector that underwent
cannibalism, and has relic density set by the freeze-out of two-to-two
annihilations. We identify three novel phases, depending on the behavior of the
hidden sector when dark matter freezes out. During the cannibal phase, dark
matter annihilations decouple while the hidden sector is cannibalizing. During
the chemical phase, only two-to-two interactions are active and the total
number of hidden particles is conserved. During the one way phase, the dark
matter annihilation products decay out of equilibrium, suppressing the
production of dark matter from inverse annihilations. We map out the distinct
phenomenology of each phase, which includes a boosted dark matter annihilation
rate, new relativistic degrees of freedom, warm dark matter, and observable
distortions to the spectrum of the cosmic microwave background.
