"Mhach, Mhach, beware the dark,
keep away or lose your heart."
Mhach (魔法都市マハ, Mahō Toshi Maha?, lit. Magic City Mhach) was an ancient city of magic in Final Fantasy XIV that existed in Eorzea during the Fifth Astral Era. Though traces of their legacy were present in A Realm Reborn, they were not mentioned by name until Final Fantasy XIV: Heavensward.
Advances in Mhachi sorcery were made for one reason and one reason only: cataclysmic destructive power. And the swiftest─and most dangerous─method of acquiring that power was through the enslavement of voidsent entities.
The Mhachi thrived in the Yafaem Saltmoor northwest of Mor Dhona. Little is known about them, other than the sinister reputation of its void mages, who summoned otherworldly beings to attack their enemies in the War of the Magi: the contemporary city-states of Nym and Amdapor. As a precaution against rogue voidsent, the void mages developed the Nullstone as a means to obliterate them, and kept it secure within a great pyramid in the city proper.
Using an elaborate trap, the Mhach tricked a Nymian sea galley into bringing an ampoule to their home nation containing a voidsent named Bitoso. This voidsent would unleash a devastating plague called the "Green Death," which transformed many Nymians into tonberries. In their warfare against Amdapor, they unleashed Diabolos and Ferdiad. The white mages sealed the great voidsent deep within the Lost City of Amdapor and Amdapor Keep.
The War of the Magi's deleterious effect on the realm's aether began to push the Elementals into retribution. The void mages learned that a great flood would soon engulf the world and sought to evade the calamity, the Mhachi preparing a great ark to take to the skies. To power such a vessel, the void mages bound a thousand voidsent within the vessel that included the Shadow Queen Scathach, whose presence was ultimately the Mhachi people's undoing. Fifty voidmages sacrificed themselves to reseal the rampant voidsent with the ark disappearing into the sky. The Nullstone was kept in its securely warded tomb in hopes the rising floodwaters would deny any voidsent access to the artifact.
As the floodwaters of the Sixth Umbral Calamity receded, Yafaem became swampland and the waterlogged ruins became known as the Weeping City. When the Sixth Astral Era dawned, mages were persecuted due to the turmoil that the War of the Magi had wrought. Many Mhachi survivors later founded the city-state of Belah'dia in Thanalan, in part to escape the persecution.
Though Mhach itself was no more and became a faded memory, the city and many remnants of its occupants' warfare with Nym and Amdapor would be unearthed during the upheaval of the Seventh Umbral Calamity. The lost Void Ark can be seen as a ghost ship drifting in the Sea of Clouds above Abalathia's Spine. When Diabolos is freed from his confines in Amdapor, he reaches the Void Ark and retrieves Scathach's prison, and sends Forgall and the recently freed Ferdiad to the Weeping City to obtain the Nullstone, so that they could act without fear of annihilation. Diabolos underestimated how severely his time sealed away in Ampador had weakened him, however, and his plans are undone, at no small cost, by a coalition of adventurers and sky pirates, and with the assistance of an ancient Mhachi familiar.