The Time Devourer is the final boss of Chrono Cross.
After Lavos was defeated, he was most likely sent to the Darkness Beyond Time. There, he intercepted Schala, who came in from the Ocean Palace Incident; he then merged with her to form a new being, the Dream Devourer, who, in its maturity, became the Time Devourer, a being capable of consuming all space-time. Belthasar planned every event in the game so that Serge could get his hands on the Chrono Cross artifact and destroy the Time Devourer. Within the confines of the Time Devourer rested both Schala, imprisoned for millennia in his body, and the true Dragon God, the biological supercomputer defeated by FATE and its fighter jets in year 7000 B.C. Lavos was using the Dragon God in its separated forms in order to retrieve the Frozen Flame, a piece of him that splintered off when he crashed to Earth. The Frozen Flame acted as Lavos's eyes and ears during this entire endeavor. He very nearly succeeded in his plan but was brought to justice by Serge and his party. The Chrono Cross affected the Unification of Dimensions, and the separation of Schala from Lavos resulted in his death at last.
The Frozen Flame was shown to serve as an uplink to Lavos; before the fight with the Dragon God in Chrono Cross, the characters speak directly with Lavos through the Flame. Lavos's responsibility for humanity's final evolution is a basis for the Dwarves' and Demi-humans' criticism of humanity for ruining the environment and planet. Lavos is suggested to be a very evolved state of humans, who have agglutinated and consumed all life. In the Chrono series, it is implied that the material, constantly decaying part of humanity—derived from Lavos's evolution—is evil, while the eternal, divine essence is good. Nonetheless, humans have the chance to prove themselves before the Dragon God and Demi-humans in Chrono Cross. They may be the progeny of Lavos, but they retain free will.
Signs point to another overarching storyline that involved Serge bonding with the Time Devourer itself; in this scenario, Serge, if he did not defeat the Time Devourer with the Chrono Cross, would ultimately bond with it and cause the Devourer to mature, granting him the power to consume space-time. Chrono Cross Ultimania mainly supports this notion, claiming that the Arbiter of the Frozen Flame must mediate between his fellow lifeforms and Lavos, or else he will inevitably bond with the creature and spell destruction for other humans. Ultimania also postulates that Schala was an Arbiter of the Frozen Flame while it existed in Zeal, and that is how she came to bind with Lavos. Apparently, the Time Devourer needed enough power before it could reach maturity, and Serge would have been the final piece in the puzzle. Quotes in the game also seem to hint at this possibility; three come to mind specifically. A demi-human in Marbule states that "anyone who touches that Flame will become a different being." Lynx, at Viper Manor, warns Serge that "there shall be a deep enmity between you and the world" once destruction occurs. Lastly, Belthasar notes that the Arbiter will gain "extraordinary powers...by binding with the new seed of destruction, the Devourer of Time." However, this storyline is not directly stated or implied in Chrono Cross, and neither has series creator Masato Kato confirmed it himself. For the time being, it remains speculation made by the Ultimania authors. Considering that in the same book, they contradict another canon (such as stating the Prometheus Circuit is not Robo, when Masato Kato noted that the circuit is Robo), the veracity of Ultimania is arguable.