The Widow Harun
Thay is one of the nations of The Eastern Realms, a magocracy that is considered one of the daughter nations of Mulhorand. Historically, Thay has threatened all its neighbors with conquest; in more recent times, the nation became an exporter of cheap and ubiquitous magic items; and today Thay is dealing with an existential threat from The Tuigan Empire. The Fencing of Thay began shortly after The Demonwar, when Jebe Khan and his descendants persuaded various regional powers to form a united front aimed at containing The Red Wizards. In the years following, the combined forces of Mulhorand, Aglarond, Rashemen and Thesk joined the ordus of the Tuigan in overrunning all the lowland possessions of Thay. The Red Wizards lost all four of their eastern border fortresses in The Sunrise Mountains, as well as the tharchs of Priador, Surthay and The Alaor, plus Thazalhar and the twin cities of Nethentir and Nethjet. At least one Zulkir was slain in battle in this period. Szass Tam appeared in person, but was forced to retreat from the battlefield by a cadre of high-level Gezerite clerics. Many dozens of clerics of Kossuth are now laboring in The Camps of the Godslaves. Led by Mulhorand, the neighboring powers also imposed a naval blockade, eventually causing famine. More importantly, the Thayan leaders were cut off from their lucrative enclaves throughout The Western Realms. Many of these enclaves were then simply seized by the rulers of their host cities; others were destroyed by The Harpers or similar do-gooders, but a few have survived as autonomous municipalities. Thay is now much reduced in wealth and influence; the population has crashed due to hunger and the disruption to law and order that ensued after the catastrophic defeat of The Red Wizards by their combined enemies.
Thay is described in much more detail in the Forgotten Realms core books and supplements. Please consult these if you wish to learn more.