Long ago, Cait Sith was a familiar of the Mhachi High Voidmage Cessair, who foresaw the flood of Sixth Umbral Calamity due to disruption on the land's aether caused by Mhach's destructive campaign against Nym and Amdapor in the War of the Magi. Her people attempted to evade the calamity by creating the Void Ark with a thousand sealed Voidsent powering it, only for it to end in disaster as not even Cessair can control the Voidsents' leader Scathach. Eventually, Cessair and 53 other void mages had to sacrifice themselves to keep Scathach and the other Voidsent sealed away with Cait Sith the only survivor. He was tasked to ensure the Void Ark would not be disturbed as it drifted in the Sea of Clouds for centuries.
In the aftermath of the Seventh Umbral Era, the calamity caused by Bahamut undid the seal that bound the Voidsent Diabolos in the Lost City of Amdapor. Though defeated by the Warrior of Light, Diabolos escapes and makes his way to the Void Ark where he regains his strength while removing the seals on his fellow Voidsent. When the Warrior of Light is hired by the sky pirate Leofard to search the Void Ark for treasure, Cait Sith makes his presence known when Diabolos retrieves the Queen's Coffin that contains Scathach and takes it through a voidgate. He tells his story while imploring the sky pirates help him track down the Queen's Coffin before Scathach is released.
Cat sìth (meaning "faerie cat", pronounced "Kat Shee") is a fairy creature from Celtic mythology, said to resemble a large black cat with a white spot on its breast. Legend has it that the spectral cat haunts the Scottish Highlands.
The legends surrounding this creature are more common in Scottish folklore, but a few occur in Irish. Some common folklore suggested that the Cat Sìth was not a fairy, but a witch that could transform into a cat nine times.