The Hellmouth, also referred to as "the aperture" was a space-time "bridge" between realspace and Otherspace. Constructed by the Charon Dominion, the Hellmouth was a breach between dimensions through multiple overlapping quantum fields, and was at least partially forged by the spacial rift anomaly that began in the galaxy, known as the Kathol Rift.
Possibly because of its connection to the Kathol Rift, the creation of the Hellmouth caused the implosion of planet Kathol itself, which became the physical location of the Hellmouth. Although considered to be a transdimensional wormhole, the Hellmouth had as strong oscillating gravitational fields, akin to a black hole, but repelled matter instead of attracting it. Approach and passage through the Hellmouth was believed to be possible only via protomatter, a natural matter substance found in Otherspace that shares amalgamated characteristics with antimatter, dark matter, hypermatter and negative matter.
Because the Charon Dominion utilized protomatter-powered phase engines as opposed to hyperdrives, transit through the Hellmouth was almost entirely restricted to the Charon Dominion. However, at some point after the Extant War began, the Sith Order managed to acquire protomatter and a phase engine from the Charon Dominion, and used it to spearhead a joint invasion into Otherspace. Due to its expense and difficulty in creation, utilization by the galactic powers was more limited than the Charon Dominion, who ultimately became the primary users of the Hellmouth during the war.