The Voidbrood later spread to many other Imperial worlds in the region. At some point after the opening of the Great Rift, it became an unusual ally of the Tyranid Hive Fleet that destroyed the Agri-world of Taurensi IX.
This faction of the Voidbrood aided the Tyranids in consuming the rest of the Taurensi System, believing that they were aiding their long-awaited alien gods in their pursuit of divine ascension. They never realised that the Hive Mind saw them simply as useful pawns and potential extra bio-matter to be consumed if the Hive Fleet's reserves ran low.
In the late 41st Millennium, the wreckage of the Imperial freighter Pegasine, destroyed by the Lance strike of an Ordo Xenos corvette, spiralled through the skies of the Frontier World of Hopefoster. Most of the debris burned up on entry, but the largest section of the battered vessel landed more or less intact.
After long solar weeks spent healing, a single Purestrain Genestealer survivor awakens in the wreckage. It becomes the Genestealer Patriarch of the Voidbrood, and after a Terran century of unfettered expansion, its cult rises up to overrun the planet entirely.
After the opening of the Great Rift, in the midst of the worldwide invasion of the Agri-world of Taurensi IX, a vast swarm of Tyranids descended to annihilate the planet's Astra Militarum defenders, the Hive Mind's creatures begin to consume the planet -- but do not turn upon the Genestealer Cultists of the Voidbrood who helped prepare the way for their conquest.
Perhaps the strange departure from the Tyranid consumption modus is due to a psychic imperative from the cult's Genestealer Patriarch, or perhaps it can be traced to a flaring of the Cicatrix Maledictum high above -- but to the cultists themselves, it is simply another aspect of their divine ascension. They take to the skies in bulk freighters alongside the vast, sky-spanning bio-ships of the Hive Fleet, and are ignored entirely by the Tyranids, much as the tiny scavenger-fish is ignored by the goreshark with which it shares a symbiotic bond.
The rest of the Taurensi System is conquered by an invasion of zealous, frothing Genestealer Cultists and the trillions-strong swarms of the Tyranids that they think of as their allies -- when in truth, the synapse creatures of the Tyranid hordes see the cultists as little more than ambulatory biomass to be consumed whenever the need arises.
- Codex: Genestealer Cults (8th Edition), pp. 37, "A Single Seed," 39, "The Heart of the Voidbrood"