The Halo Stars is the name given to a formation of stars on the very edge of the Milky Way Galaxy, representing the last stellar clusters to be encountered before one enters the eternal, frigid night of the intergalactic void.
The Tharkad Expanse (named for the planet of Tharkad, which is the last League outpost before the Passage) refers to a number of Halo Stars systems bordering the rimward edge of Star League space, a dangerous and unexplored region that is subject to occasional League-funded expeditions to map out these systems. Many expeditions never return, something which contributes to the malignant reputation the Expanse has in the eyes of many League spacefarers. The Expanse is accessed through the Tharkad Passage, a dangerous hyperspace route between a number of black holes that have claimed many would-be explorers before they even reached the Expanse. While largely unmapped still, the Expanse has been subject to a few colonization attempts over the decades, with a few small colonies established in the region.
For decades, League explorers have braved treacherous hyperspace lanes to venture into the Expanse, but their efforts have barely begun to uncover its secrets. Footholds have been built close to the few semi-stable hyperspace routes into the area. Here, resource-rich worlds are exploited, ancient xenos ruins are excavated, and efforts at establishing colonies are attempted upon sheltered worlds. This effort has been enough to shower wealth upon the fortunate -- and make corpses of the rest. The gateway to the Expanse is littered with broken vessels and tales of the vanished.
The hyperspace paths of the Tharkad Expanse are treacherous in the extreme, and the partially explored regions of the Expanse are islands of League activity amidst a vastness of empty space. Navigating is a far cry from traversing the well-established hyperspace routes of League space. Most of the Expanse is unknown, hyperspace routes scarcely mapped by the efforts of those that have survived travelling in the cold dark. The explorer who ventures into the Expanse risks his life and ship on luck and on the quality of what little information he has gleaned from those gone before.
Beyond this small zone at the edge of civilization lie truly dark and dangerous voids, rife with rumoured terrors, undiscovered stars, and worlds of xenos never before seen by the eyes of man. There are no defined hyperspace routes, no safe paths through the cold void. These regions are most reviled by League explorers, for very few survive attempts to brave the eternal dark.