Antoronto is a Phantom Monster Warrior belonging to the Phantom Mutant Beast Corps, the primary antagonistic force in the latter part of the series. This creature was originally a man-eating ant from the Anchron region, a desert area of the Monster World, that was subsequently captured and enhanced through military modification. Antoronto is distinguished as a master of sand, wielding this element as his primary weapon in combat. His abilities include generating powerful sandstorms to disorient and trap enemies, creating solid barriers of sand for defense, and compressing sand into massive boulders to hurl at opponents.

Antoronto is driven by the mission to eliminate the protagonist, Kamen Rider BLACK RX. To achieve this, he attacks civilians to lure his target into a confrontation. His personality is that of a confident and aggressive warrior, as evidenced by his battle cry, though his arrogance can be his undoing. Initially, he engages Kamen Rider BLACK RX but is forced to retreat under the Rider's fierce assault. Following this failure, he receives a powerful enhancement from Gedorian, the leader of the Four Great Commanders of the Crisis Empire, who seeks to make him the strongest warrior in the corps. This upgrade grants Antoronto the new ability to create a gigantic termite mound on the Earth's surface, which he uses as a trap.

In his most significant action, Antoronto interrupts a critical duel between the resurrected Shadow Moon and Bio Rider, one of RX's alternate forms. By generating a massive tornado that engulfs both combatants, he successfully seals them inside his newly created termite mound, aiming to destroy them both at once. This act serves his masters, the Four Great Commanders, who fear Shadow Moon might usurp their leadership. However, his success is short-lived. The Rider Machine, Ridolon, destroys the mound with a charge, allowing Bio Rider to escape. Seizing the moment of Antoronto's hesitation, Kamen Rider BLACK RX strikes him with the Revolcrash, his signature finishing move, causing the monster to explode and be destroyed. After his death, the Four Great Commanders flee the scene and are later punished by General Jark for squandering a golden opportunity to defeat their enemy.