  We propose a new thermal freeze-out mechanism for ultra-heavy dark matter.
Dark matter coannihilates with a lighter unstable species, leading to an
annihilation rate that is exponentially enhanced relative to standard WIMPs.
This scenario destabilizes any potential dark matter candidate. In order to
remain consistent with astrophysical observations, our proposal necessitates
very long-lived states, motivating striking phenomenology associated with the
late decays of ultra-heavy dark matter, potentially as massive as the scale of
grand unified theories, $M_\text{GUT} \sim 10^{16}$ GeV.
