12,000 B.C., also known as the Dark Ages, is one of the eras in the role-playing game Chrono Trigger. It's known as the Dark Ages because, even though it seems like a peaceful and magical place, there is an evil that looms and eventually destroys almost everything. The playable character Magus is originally from this era.
After having jumped through the Time Gate in the Lair Ruins in 65,000,000 B.C., Crono, and his friends find themselves in a new era. It's 12,000 B.C., known as the Dark Ages, where a magical kingdom floats high above the surface of the frozen planet.
Traveling through Skyways and Land Bridges, they make it to the floating kingdom where the people there, calling themselves the Enlightened Ones, are able to use magic. In talking to the people, Crono's party finds out that Queen Zeal is channeling Lavos' power through the Mammon Machine. Knowing that Lavos is the root of the inevitable destruction of the planet, they want to put a stop to Zeal's plans.
They learn that Marle's pendant was actually Schala's pendant in this era, and it's a magical pendant that can be powered up by the Mammon Machine. So they power up the pendant to use it to enter the Queen's chambers. As they enter, the mysterious Prophet warns Zeal that Crono can be dangerous. So Dalton sits Golem on them.
After Crono's party defeats Golem, they are imprisoned by Zeal, only to be rescued by Schala and her kid brother Janus, who know what their mother, Zeal, is doing is wrong. The Prophet means to kill Crono's party, but Schala wants their lives spared. So they all travel back down to the Time Gate, send Crono's party through it, and the Prophet forces Schala to magically seal the Gate for good.
Now in possession of Epoch, Crono's party returns to 12,000 B.C. where they head to Terra Cave. In talking to the people there, called the Earthbound Ones, Crono learns that Queen Zeal locked up the Guru of Life on the Mountain of Woe.
So they head up the magic chain that tethers the floating mountain and defeat the monstrous Giga Gaia to rescue the Guru. It's then that they realize the Guru of Life is Melchior. Since the Giga Gaia was destroyed and the seal had been broken, the mountain was collapsing. They all got off the mountain safely before it fell into the water.
Crono's party returns to Terra Cave with Melchior.
Crono's party then takes the Skyways and Land Bridges back up to Zeal Palace where they confront Dalton. After the battle and Dalton is defeated, he opens a portal and travels to the Ocean Palace deep under the water. So Crono's party follows him through the portal.
While the Mammon Machine is channeling Lavos' energy, Crono stabs it with the Ruby Knife. Energy is discharged and they all find themselves on a different dimensional plane where Lavos finally emerges. The monstrous Lavos easily defeats Crono's party and leaves them within an inch of their lives.
It's then that the Prophet reveals himself to be Magus in disguise. After the battle at his castle in 600 A.D., the portal transported him here to the Dark Ages. Magus then attacks Lavos but is also defeated.
Lavos then tries to steal the life force from Crono's party, Magus, and Schala, while Zeal stands back and laughs. Mustering some strength, Crono tries another attack, but Lavos obliterates him, leaving no remains.
The party, Schala and Magus find themselves transported back to the Ocean Palace...without Crono. Before Lavos' energy destroys the underwater fortress, Schala uses the last of her power to transport them, and Magus, to safety.
Lavos' energy is so strong that it destroys the Kingdom of Zeal floating high above the planet. The remains of the Kingdom fall to the ocean below causing a huge tidal wave to wash over most of Terra Continent.
Now in the Last Village on what remains of Terra Continent, they see a few Enlightened Ones survived the collapse of the Kingdom of Zeal. Heading up to the Commons, Dalton shows up and declares himself King since Zeal had disappeared. He also takes the party as prisoners.
They are held captive aboard Dalton's Blackbird high up in the air. Fighting their way through the massive plane, they see flying next to them, Dalton in the Epoch. Dalton had attached wings to it and claimed it for himself. The party inevitably defeats Dalton, destroys the Blackbird and takes back Epoch.
Magus then agrees to join the party in an attempt to finally defeat Lavos once and for all. As they board the Epoch together, the Black Omen suddenly appears in the sky.
The party then travels to The End of Time.