When the Mammon Machine was activated, it began absorbing too much power. Queen Zeal and Schala along with the three Gurus were the only ones at the Mammon Machine's activation. The Gurus warned about a dimensional vortex, a powerful human cannot control, and they would be dragged into a rift in time, yet the Queen persisted in activating the machine. Soon, Janus entered the room for Schala. Suddenly, the six were transported in front of Lavos. Gates then appeared and transported Belthasar, Gaspar, Melchior, and Janus to different time eras. Belthasar was sent to the Future, Gaspar was sent to the End of Time, Melchior was sent to the Present, and lastly, Janus was sent to the Middle Ages.
In the Future, Belthasar created a machine that could travel through time, naming it the Epoch. When Crono's party enter the Keeper's Dome, they find fragments of information that reveal Belthasar was sent to the Future by a great disaster which befell his land and that much to his surprise, he has learned that Lavos was not a product of the Antiquity. Instead, Lavos must have fallen from the sky in a bygone age and had burrowed deep into the planet's core where he began to feed upon its energy to gain strength. He goes on to reveal that Lavos's form was revealed briefly when he was summoned forth by Magus in the year 600. In the Apocalypse, Lavos emerged to claim the surface and now he has taken Death Peak as his nest, where his miniature spawn now flows from the mountaintop. He also states that Lavos is a parasite of gargantuan proportions whose host is the planet itself.
Trapped in the future, he endeavored to continue his study and observation of Lavos. However, his mind began to lose its sanity, so he left records of his knowledge behind, which Crono's party finds in the Keeper's Dome, along with his greatest invention, the Epoch.
Crono's party discovers the Epoch, and then a Nu enters the room. It reveals itself as Belthasar, who says that he copied his memory into the creature. He then explains how to control the Epoch.
Later, when Crono is killed by Lavos and the party obtains the Chrono Trigger, the party returns to the Keeper's Dome for information on how to revive Crono. He tells them that Death Peak harbors a power capable of restoring the slain to life, though the power to reverse death is not one easily invoked. The need must be great, and the person's existence of the utmost importance. Also, they will need a double, a doll identical to the person in every detail. He tells them that the magician Norstein Bekkler could make one for them. When they obtain a doll that resembles Crono from Norstein Bekkler, they return to Belthasar. He tells them that he has implanted one last program in the Nu's memory banks that will help them climb Death Peak.
Belthasar releases three Poyozo Dolls, which he says will aid the party on Death Peak. He then says that the construct has reached the end of his final program and asks them to turn it off.The Guru of Time and the Prophet. When first encountered, he is friendly and is accompanied by the more hostile Marcy. Later during the attempted escape with Riddel from the Porre army, Belthasar assists in Serge's escape, only to have the party instead attacked by Grobyc.
During the events of Chronopolis, it is revealed that Belthasar was the one in charge of Project Kid. As the keeper of Chronopolis, Belthasar is the guardian of the Frozen Flame, protecting it from the dragons.
Belthasar caused the Time Crash, sending Chronopolis into the distant past, stranding him in 2300. From the future, he made his own Neo-Epoch, and traveled back to 1020 in order to direct Serge, and fix Project Kid. This Neo-Epoch can be viewed in a secret room in Viper Manor.
Belthasar gets his English name from the traditional names given to the Magi (Gaspar, Melchior, and Balthasar) who brought gifts of gold, frankincense, and myrrh to the infant Jesus in the Christian tradition.