The Riftworm is colossal in size, measuring at 8-10 miles long and half a mile in width. The full details of its biology are unknown, but it has multiple hearts and its innards are expansive enough that they can be explored by humans, as proven by Marcus Fenix and his Delta-One squad. Within the Riftworm's body are parasitic creatures called Nemacytes which aid in the Riftworm's digestion. They also serve as antibodies, of sorts, hunting down any swallowed creatures that remain alive inside the beast's innards. The excrement of the Riftworm also acts as a potent fertilizer, enriching Sera's soil. Because of this, the Kantus Priesthood, the principle religion of the Locust, revere the Riftworm as a life-giving deity.
In the fourteenth year of the Locust War, the Riftworm was awakened following the COG's Lightmass Offensive. With the great creature stirred by the detonation of the Lightmass bomb, the Locust high priest Skorge attempted to direct the Riftworm's fury and use it as an asset in the Horde's war against humanity. The monster began burrowing around the Jacinto Plateau, sinking entire cities surrounding the COG's primary stronghold. The COG retaliated by commencing Operation: Hollow Storm, a direct invasion of the Locust's home territory.
During Hollow Storm, Marcus Fenix and his squad witnessed the awesome destructive power of the Riftworm firsthand and found themselves being swallowed by the beast. Trapped inside the behemoth, Delta Squad attempted to stop the Riftworm's rampage by killing it from within, venturing through its body and seeking out its hearts. Using the chainsaw bayonets on their Lancer assault rifles, Delta Squad cut the arteries of each of the Riftworm's hearts, causing it to go into cardiac arrest. The Riftworm then surfaced as it died, spewing gallons of blood into the air. Delta Squad almost drowned in the creature's blood themselves but were able to cut their way out of the monster with their chainsaws.