Mummies exist in the Darkhive as a peculiar side effect of certain regal Panaho funeral rites. Occasionally when one of the leaders of the Panaho becomes too intrinsically connected to the fungal symbioties in their systems, their whole body starts to slow down. They become one with the fungus and literally seize up as the fungal life threads its way through their veins and muscle tissue.
Such regal elders either go insane as they find it harder to remain mobile, or they spend longer periods in meditation to develop otherwise unknown heights of psychic awareness. Those who go insane are often set slowly wandering the depths of the hive where they might confront mutants and monstrous creatures in one last civic duty for their people. Those who develop psychic powers are carefully bandaged to avoid injury as their mind wander the astral realm. Ghosts have often claimed to encounter the astral forms of these mummies, but they fear these powerful beings. The little news about the mummies which filters through to those still living speaks of grand psychic battles between savage races of monsters. These monsters may be the driving forces behind the Shellbrood, the creatures who use demons and devils as their servitors, or even the creatures who originally created the Darkhive itself. Transcending to take part in this eternal battle beyond tie and space is considered one of the greatest honours for the regal elders of the Panaho, and something that lower ranked Panaho believe is beyond their grasp (but ironically, there may be just as many lower ranked Panaho symbiotically waging the eternal war from their moldering corpses in the middle of fungal gardens...they just don't fight as long as their bodies gradually give way around them).
Eventually, most Panaho mummies become solid fungus, the entirety of their bodies used to nourish new life over the course of centuries. Some gardener priests tend mummies, pruning the fungus to preserve the remaining flesh and bone for as long as possible. These gardener priests are often granted blessings by the mummy over the course of generations. These blessings become ever more powerful when they are granted (as the mummy gains power at levels typically considered akin to a spirit or deity), but as the mummies consciousness drifts further into the deep realms, it's concept of time becomes ever more tenuous and those blessings become more sporadic.