Forgotten Realms - Rites of Ascension
The nation of Thay came about when a sect, calling itself the Red Wizards, declared its freedom from the god kings of Mulhorand. The sect’s center of strength was in the northern provinces where the natives did not have the inbred reverence for the god-kings. After razing the city of Delhumide they declared themselves the free nation of Thay in 922 DR. Since then Mulhorand has had to defend themselves, quite successfully, against two invasions from Thay.
The armies of Thay have marched on Aglarond and Rashemen many times and folk fear that the time will soon come when the Zulkirs mobilize again. On the reverse, few invaders have ever overcome the great mountains girding the Plateau of Thay, and the secretive and suspicious Red Wizards raise barriers just as formidable to discourage travelers from venturing into their lands.
In 1375 DR, Szass Tam set in motion a series of events such as the murder of the zulkirs of Transmutation, Druxus Rhym, and Evocation, Aznar Thrul, crafting an undead army with the atropal, Xingax, from the bodies of slaves and prisoners of war, portraying a failed unauthorized Thayan invasion on a Rashemi town by the Tharchions, Azhir Kren and Homen Odesseiron, as a Rashemi invasion onto Thayan soil stopped by the great casualties of Thayan soldiers and his ‘timely’ rescue of the invading force. All of these actions created a climate of fear within the nation of the Red Wizards and endearing him to the people as a savior that could end the country’s woes.
His plot was uncovered by his former confidant Dmitra Flass, who put the puzzle together and alerted the remaining zulkirs, obviously excluding the lich. When Szass tam proposed to make himself ‘temporary regent’ to end all the suffering of the nation and crush the undead forces and other such troubles, the five of the six zulkirs voted against his proposition with only Yaphyll, zulkir of Divination, abstaining to vote. Szass Tam immediately had the Order of Necromancy under his control alert the common folk that the beloved hero of Thay had be denied from ending their suffering. Riots erupted in the streets, which he also used to his advantage, taking control over garrison commanders and summoning demons to kill the rioters. The appearance of the Tanar’ri did nothing to behoove the reputation of Nevron’s Order of Conjuration, as they believe his servant had summoned the demons to fight. Szass Tam appeared all over the city to quell the fighting, endearing him to the people even more.
Soon afterword, much of the undead force had been destroyed and the nighthaunt and Angel of decay leaders had been obliterated along with much of the Burning Braziers, the later thanks to the rigged magical torches he had given them, Szass tam openly declared himself ruler of all Thay, thus declaring war against the remaining zulkirs of Thay.
Thayans believe that the gods send luck to the strong and resolute, not to the gentle and compassionate.
(The above info was provided by http://forgottenrealms.wikia.com/wiki/).