Tsien Chiang breaks her oath

The seminal event that caused I'Cath to slip into the Mists occurred in 732 BC. In one version, the Seminal Event occurred after many years of imprisonment for Nightingale in the Tower of Broken Promises and the deaths of many of her would-be suitors at the hands of Tsien Chiang, her evil mother. When Nightinggale morally questioned the actions of her mother for the fourth time, [Tsieng Chiang beat and tortured her. Tsien Chiang's ultimate goal was to utterly break her daughter's corporeal body. When Nightingale screamed for help, an ominous fog| began surrounding her Palace of Bones. Believing the gods were trying to deliver her to Hell, Tsien Chiang defiantly proclaimed that the Nightinggale was bound to her until complete destruction. Furthermore, Tsien Chiang gods would never never see Nightingale unless she was delivered the secret of the number four. When the Emperor of I'Cath tried to intercede, she struck him down, undoing the sacred oath of loyalty she had sworn. The Mists enveloped her home and delivered her to a tiny Island of Terror.

In Tsien Chiang's shortened profile given in Domains of Dread (beyond referring readers to IoT), the seminal event of I'Cath is the imprisonment of Nightingale in the Tower of Broken Promises.