Lavos is the primary antagonist of Chrono Trigger. It is a gigantic, incredibly powerful alien of unknown origins. Originally, it was named by Ayla in the ancient language (La means fire and Vos means big). Lavos first came to earth in 65,000,000 B.C., which was when it dug itself deep into the earth. While it was in this state of dormancy, it absorbed DNA of the planet. The idea is that Lavos altered the earth's growth to make the best possible DNA for it to absorb and pass onto its spawn.
When Lavos first landed, he crashed directly into the Tyrano Lair and wiped out the intelligent race of dinosaur-like creatures known as Reptites. This allowed the humans to rise to dominance. In 3,000,000 B.C., humans came into contact with part of Lavos (the Frozen Flame), which rapidly accelerated the evolution of humans, granting them with incredible intellectual potential and magical abilities.
In 12,000 B.C., the magic-using humans built the kingdom of Zeal. This kingdom was known for many incredible feats of magic and science, such as the Time Egg, Blackbird, Epoch, and Masamune. Lavos was hailed as a god by some people, most prominently by Queen Zeal. She was corrupted by the want and need for Lavos' incredible power. She built the Mammon Machine, a machine meant to draw power from Lavos. However, when they tried to harness his power, the kingdom of Zeal was destroyed, Schala (Queen Zeal's daughter) was killed and Janus (Queen Zeal's son, and later called Magus) was sent forward in time.
Because the kingdom of Zeal had been destroyed, people who could not use magic could now rise to power and form empires. Lavos didn't emerge until 600 A.D. when Magus summoned it. Then, in 1999 A.D., Lavos finally erupted and nearly wiped human life from Earth (the few humans that were left were being slowly murdered by the corrupt supercomputer Mother Brain). This day became known as the Day of Lavos. Crono, Lucca, and Marle discovered this when they reached 2300 A.D. and decided to keep this from happening.
The Final Battle
The battle against Lavos takes place in three primary stages. The first, which is fought against its outer shell, is a boss rush of sorts, as Lavos emulates the behaviors, attack patterns, strengths and weaknesses of numerous bosses fought over the course of the game. Once the outer shell has been defeated, the player's party must then enter the shell, which contains an inner being referred to simply as Lavos, followed by a battle with the Lavos Core.