Following his death caulder became this. The warband slays him, sending him to respawn. He needs eldritch energy to do so, but can otherwise respawn infinitely. I think these two quotes are interesting. This is after the building is destroyed. This is a bit later on. So we know that Caulder's soul was bound to the mausoleum guardian and it needed eldritch energies to regenerate. Following its final destruction Caulder's soul would be free. What would happen if the Mausoleum guardian survived the fall of the building is unknown, but its possible it would be fine but no longer able to respawn without a new source of eldritch energy. Assuming that's the case, its possible that after Richter left Caulder's bones the first time the guardian managed to gather just enough power to resurrect itself and leave. Alternatively the bones were blown away by the explosion, explaining them being missing later on. So there is at least a chance that a rogue mausoleum guardian is wandering around out in the tunnels, trapping Caulder's soul and using his skills.