But then one day about twenty years ago, humans stumbled across the entrance to that hidden realm...
The Raid on the Esper World is a battle in Final Fantasy VI that takes place 16 years before the start of the game. Though the Gestahlian Empire technically lost by being repelled from the Esper World, it nevertheless achieved its aim of capturing several espers. This paves the way for the invention of Magitek and, in turn, the First, Second and Third Gestahlian Campaigns.
Two years prior, a human named Madeline enters the Esper World and falls in love with Maduin, an esper. Two years later Madeline gives birth to a baby, whom they name Terra. Many espers disapprove of the relationship, but Maduin is protective and also has the support of the Esper Elder.
Two years later, an Imperial force led by Emperor Gestahl personally stumbles across the Esper World. Gestahl promises riches to anyone who captures an esper, and soon Imperial soldiers pour into the Esper World.
As the main focus of the Imperial soldiers is to seize as many espers as possible, it is likely that few if any espers got killed as a direct result of the raid. It is unknown to what extent the espers resisted the invaders, but as they were no match for the superior Imperial Army, and as confronting the attackers would mean exposing themselves to captivity (just as the Imperials wanted), they focused on falling back to avoid being taken prisoner.
Realizing they are facing a battle they cannot win, the Esper Elder decides to use his magic powers to expel the invaders and raise a barrier, sealing off the Esper World. Maduin protests, saying the Elder will use up his powers and die, but the Elder is prepared to sacrifice himself for the greater good. As the spell is cast, Maduin learns Madeline has fled through the gate with Terra due to other espers blaming her for the attack. He chases after her and the two end up critically wounded on the wrong side as the barrier is erected.
Emperor Gestahl is initially furious that the espers had halted the raid, but changes his mind when he finds Terra, personally seizing her from Madeline who soon dies from her wounds. While Terra becomes subject to experiments and is raised to become a living weapon, the captured espers are placed in the Magitek Research Facility to be drained for their powers. This is how Magitek is invented, setting the groundwork for the outbreak of the Third Gestahlian Campaign some sixteen years later. Hence, although technically losing the battle by being expelled, the raid was a huge success that defined the fate of the Gestahlian Empire, allowing it to become the dominant power of the world. To the "victorious" espers, trauma and loss was the only result of the invasion.
Ramuh with four espers manage to escape the Magitek Research Facility, during their escape, Kirin, Siren, and Cait Sith fell while Ramuh took their Magicite so they didn't fell into the wrong hands. Ramuh ends up hiding in Zozo.
In the GBA and following versions, prior to the Returner's infiltration of the Magitek Research Facility. Four espers had escaped, considering the man in Nikeah is absence in the original versions. He most likely refers to the four espers introduced in the GBA version: Leviathan, Diabolos, Cactuar, and Gilgamesh.
Eighteen years later, when the barrier between the Human World and Esper World is removed, the espers try to save their friends who were captured in the raid. When they learn they are already dead, they enact revenge by decimating Vector; it is later revealed they lost control of their powers when crossing into the Human World. The espers would later be wiped out completely in the Battle of Thamasa.