  The quantum chromodynamics axion with a decay constant near the Grand
Unification (GUT) scale has an ultralight mass near a neV. We show, however,
that axion-like particles with masses near the keV - PeV range with GUT-scale
decay constants are also well motivated in that they naturally arise from
axiverse theories with dark non-abelian gauge groups. We demonstrate that the
correct dark matter abundance may be achieved by the heavy axions in these
models through the misalignment mechanism in combination with a period of early
matter domination from the long-lived dark glueballs of the same gauge group.
Heavy axion dark matter may decay to two photons, yielding mono-energetic
electromagnetic signatures that may be detectable by current or next-generation
space-based telescopes. We project the sensitivity of next-generation
telescopes including $\textit {Athena,}$ AMEGO, and e-ASTROGAM to such decaying
axion dark matter. If the dark sector contains multiple confining gauge groups,
then the observed primordial baryon asymmetry may also be achieved in this
scenario through spontaneous baryogenesis. We present explicit orbifold
constructions where the dark gauge groups unify with the SM at the GUT scale
and axions emerge as the fifth components of dark gauge fields with bulk
Chern-Simons terms.
