The Riftworm was a massive worm that can be controlled, much like the Rock worms, by the howls and shrieks of the Kantus, in particular their leader, Skorge. During Operation: Hollow Storm, it is used by the Locust to create holes underneath the cities around Jacinto to sink it. Delta Squad sees Skorge on his Hydra ordering the beast to sink Ilima.
A Locust Tablet which Baird tries to translate, suggests that the Riftworm was 'awoken' by the Lightmass Bomb which explains why the Riftworm hasn't been seen or heard before by the COG. It also seems that Skorge, as leader of the Kantus priests, is the one that directs the Riftworm where to go.
The worm is able to eat all manner of objects including rock, vehicles and even entire buildings and digest them. It also has thousands of parasites living inside called Nemacytes that can attack anything left alive in its stomach.
During the Battle of Ilima the Riftworm swallows Delta's King Raven as it attempts to evacuate them from the ruins of Ilima, an event which causes Carmine to fall into its mouth away from the rest of the team. The chopper is subsequently swallowed by the Riftworm, and Delta presumably ends up somewhere in the Riftworm's gullet. Delta then makes its way through, avoiding the dangers of its stomach and teeth, and finds Carmine mauled by stomach parasites who then dies in the company of his squad mates. After going through the gaseous intestines, Delta cuts the arteries around its three hearts using Lancers, depriving the Riftworm of blood and sending it into shock, killing it afterward. In its death, the Riftworm beaches itself on the surface of Sera, before Delta cut their way out of it, emerging in a wave of blood.