Spheres of the Multiverse
A broken world believed to have been an ancient war zone where humanity fought against an ancient unknowable evil and forever banished this evil from the Imperial Cosmos. There are stories of a graveyard planet where Titans sleep forever dreaming of an ancient empire of demigods. Some believe that an evil is growing on Tartaros, and their is postulation that the emergence of the Rift Spheres, to split the Inner Drift away from the Middle Galaxies, could have shaken the fabric of the Planes of Existence. Tartaros being on the border between Far Space and the Known Cosmos could have shifted into one of the regions of Transitive Overlay, a world of shadow and ether.
Little is actually known of modern day Tartaros is at the far reach of the Cosmos and to survive an expedition would be a substantial investment. The Holy God-Emperor of Byzantion would not fund such a journey but then again he has more stately matters to attend to.