The Grandfather's Maw was a gulf in reality connecting the far future to the deepest past. It was known to posthumanity as the Great Attractor. With a gravity well equivalent to a billion, billion solar masses, the Maw pulled in everything in its immediate area. The Maw could be used to travel outside the Spiral Politic to the space between universes.
Circa 60,000,000,000, the Maw swallowed the Hydra-Centaurus Cluster and proceeded to destroy Andromeda, the Magellanic Clouds, and the Milky Way. At least one Homeworld was predicted to be destroyed around this time.
In the 32nd year of the War in Heaven, three Arpexia timeships were destroyed while investigating the Maw.
Jendrickenses speculated that the Yssgaroth could return through the Maw. (PROSE: The Brakespeare Voyage)
Potential origins
There were several different theories on what the Maw was. House Arpexia believed the Maw to be an impact left behind by something colliding with the surface of the universe with enough force to drive space-time into itself. House Xianthellipse's studies concluded that the Maw was the result of something from the deep past pulling down on space-time, "like an antlion making a funnel in the sand". House Lineacrux, on the other hand, claimed in private to have created the Maw themselves, something which they would later deny to the other Great Houses.
House Lineacrux built [the Maw]. It was relatively simple. A timeship materialising in and out of space-time in a tight weave curve, moving with an apparent real-space velocity infinitesimally below that of light, was used to create a gravitic attraction far out of proportion to the timeship's real-space rest mass. The agglomeration of additional mass from the nearest galaxies did the rest.
Voyage into the Maw
In secret, House Lineaux organised an expedition to go through the Maw to access the void between universes and retrieve Leviathan biodata. Entarodora captained The San Grael on a doomed voyage into the Maw. When she did not return, Lineaux got Robert Scarratt to captain The Brakespeare into the Maw.
Compassion once stumbled upon a potential timeline in which one of the War-time powers harnessed the Maw as a weapon using materials gained from The Brakespeare's successful voyage into the Maw. The Maw was "fenced off" and brought to Earth, where it devoured the planet's entire history, effectively erasing the planet from existence.
Compassion mentally contacted Captain Scarratt at a point before he entered the Maw and showed him the consequences of The Brakespeare's trip going as planned. In the new version of events, The Brakespeare failed in its mission due in large part to the interference of various War factions.
Philetes and Ella Stanton escaped The Brakespeare in a damaged timeship which they were forced to abandon after returning to the Spiral Politic. The timeship was drawn to the edge of the Maw, where it was found by Philetes, Jendrickenses, and Minkosa at an earlier point in the timeline of the War. Based on data gained from the timeship, the Great Houses decided to not send any expeditions into the Maw; thus the voyage of The Brakespeare never occurred. (PROSE: The Brakespeare Voyage)