Heavy axions can arise in the context of Grand Unified theories where a confining dark gauge group unifies with the Standard Model (SM) gauge group. Depending on the strength of the axion coupling to the SM, heavy axion lifetimes can vary drastically, requiring very different observational probes. I will discuss two such probes. First, heavy axions in GeV mass range with TeV-PeV decay constants can give rise to prompt or displaced decays in beam dump and collider environments. Experiments such as LHC, DUNE, or FASER would be able to probe such axions. Second, I will show heavy axions with MeV scale masses, but with a much larger decay constant, of order the Grand Unification scale, can be a decaying dark matter candidate. Correspondingly, it can be searched for in future X-ray/Gamma-ray telescopes, for example in Athena, Theseus, and AMEGO.