Long ago, Cait Sith is a familiar of the Mhachi High Voidmage Cessair, who foreseen 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. This causes her people to attempt 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 as he is given the task to ensure the Void Ark who not be disturbed as it drifted in the Sea of Clouds for centuries.
However, 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 Adventurer, Diablos escapes and makes his way to the Void Ark where he gradually regains his full strength while removing the seals on his fellow Voidsent. When the Adventurer was hired by the sky pirate Leofard to search the Void Ark for treasure, Cait Sith makes his presence known when Diabolos managed to retrieve the Queen's Coffin that contains Scathach and takes it through a voidgate. Later, Cait Sith tells his story while imploring the sky pirates help him track down the Queen's Coffin before Scathach is released.