The Battle of Thamasa was a short and decisive battle between Imperial forces led by Kefka Palazzo and the espers in Final Fantasy VI. It takes place moments after Leo Cristophe of the Empire and Yura of the espers had agreed on a peace treaty. Kefka crystallizes the espers into magicite before invading the Esper World to claim the Warring Triad. As the anti-Imperial forces are never able to get back on their feet following their defeat at Thamasa, for them the battle is essentially the beginning of the end.
Hoping the espers could assist the Returners in their war against the Empire, Terra Branford opens the gate to the Esper World. The espers lose control of their powers once they cross into the human world, and fly off and decimate the Imperial capital of Vector.
Following the destruction the Gestahlian Empire surrenders to the Returners hoping they could help prevent the espers from causing further bloodshed. A joint Returner-Imperial force sets out to Thamasa to search for the espers, and the Returners find them in the Esper Caves. Their leader, Yura, apologises for the destruction, explaining they lost the control of their powers.
Yura and Imperial general Leo Cristophe meet face to face and apologize to each other for the destruction their irresponsibility had caused. They shake hands and agree to end the fighting, but Kefka Palazzo arrives in the town.
Supported by two Magitek soldiers, Kefka attacks Leo's troops, the Returners and espers with Guardians in Thamasa. Having recently discovered that magicite is more powerful than magitek infusion, Kefka crystallizes Yura and the two espers by turning them into magicite, one of whom attempted to break for it before being turned. Kefka then orders his troops to burn the town due to it being "too boring". It is unknown whether the locals resisted, but with no visible destruction and the Mayor of Thamasa commenting there were a few casualties among the villagers, it is possible they used their magic to hold back the Magitek soldiers. General Leo attempts to stop Kefka, but Kefka has the remaining Guardian knock him down. Leo, sickened by Kefka's atrocities, then attempts to fight Kefka, but ends up fighting a shade and is struck down after a battle between the two. Before striking the killing blow Kefka taunts Leo, including using an illusion of Emperor Gestahl to "admit" that the Emperor ordered the attack, saying he will tell Gestahl that he disposed of a traitor.
Responding to the cries of their friends in the human world, ten espers rush from the Esper World to their rescue. By now Kefka has become considerably stronger, crystallizes the espers and takes their magicites totaling to thirteen overall. While torching Thamasa, the Magitek soldiers are killed by Kefka's indiscriminate attacks.
A wounded Interceptor returns alone after Kefka's attack and the Returners believe that Shadow died during the attack. It is unknown what happened to the Magitek Armor Transport Ship and the five soldiers who were stationed on board it.
With the Esper World unprotected, Kefka and Emperor Gestahl enter it and use the power of the Warring Triad to raise the Floating Continent. Although the Returners destroy the Imperial Air Force and make the Floating Continent accessible following the battle over the Floating Continent, they are unable to reclaim the Warring Triad or prevent Kefka from altering its alignment. Kefka kills Gestahl, and as the alignment defines the shape of the world, by moving the triad Kefka causes the cataclysm wiping out both the Imperials and the Returners, marking a de facto end to their war.