The Thrassian Plague (called slug famine by some ancient texts) was an artificially-created pandemic of mass proportions that occurred in 1E 2260, and the first known historical event to happen after the Dragon Break. It is estimated that half of the population of the Iliac Bay, and perhaps even Tamriel, perished because of it. It was created by the Sload and received its name from their homeland, the coral kingdom of Thras.
History
While the reasons for its creation and the methods used to discover the culprits are uncertain, it has been established that the slugfolk of Thras released this calamity on Tamriel. Appearing in 1E 2260 (some sources state it was as early as 1E 2200), it washed upon the shores of western Tamriel, closest to Thras, and soon spread further across the mainland (going as far as Elsweyr), destroying cultures and decimating populations as it went. The Archmage Syrabane, using his powerful ring, is said to have saved many people from perishing from the disease, possibly even finding a cure.
While it's unknown when the disease fully subsided, cases of the Thrassian Plague were still being reported near Wayrest over four hundred years after its initial outbreak.
Aftermath
The Thrassian Plague had major political and cultural repercussions on Tamriel, the most notable one being the assembling of the All Flags Navy, tasked with the invasion of Thras. Forces were sent from almost every nation of western Tamriel, ancient enemies putting asides their differences to exact revenge on the Sload. Assisted by Syrabane and led by Bendu Olo, its Baron-Admiral and the king of Anvil, the All Flags Navy "sailed to Thras, slaughtered all the Sload they could find, and, with great unknown magicks, sunk their coral kingdoms into the sea." It would take many years before Thras was restored to its former glory, and its population never attempted any similar attack on Tamriel.
Other consequences of this temporary alliance were evident decades later. The western parts of Cyrodiil began to overshadow the more advanced and religiously ardent east, which eventually lead to the War of Righteousness that ended the thousand-year rule of the Alessian Order, which in turn shifted the focus of power in the Iliac Bay from Daggerfall to Wayrest.
Another dire consequence of the Thrassian Plague was the destruction of the equilibrium of the Khajiiti people due to heavy loss of life. The disease forced the survivors into roles they did not choose, polarizing their culture and forever changing their region from sixteen peaceful and co-existing states that resisted all foreign invasions into two very culturally opposed and politically divided states, Pellitine and Anequina.