The Quelling was a horrific "crusade" led by the Aasimar to see to the eradication of the Tieflings

In [insert time period] the Thrarian Covenant came to see the presence of the Tieflings as a threat to the Prime Material Plane and the Astral Sea. This was not the first time this had happened but the differences of the two races had been put aside long ago after a peace treaty was signed by the most prominant members of the two races. The treaty stated that neither race was to enter the lands of the other race nor interfere with comings and goings on. The Thrarian Covenant, only one of four powerful sects of the Aasimar culture, was appauled by the signing of this treaty but by the law of their people could not openly do anything about it. That was until the appearance of the Vaashtaanii .

The Vaashtaanii, though not Tieflings in any way, were often confused as Tieflings in part because of their vague resemblence but mostly because Tieflings had such a wide range of characteristics due to their fiendish heritage. Vaashtaanii often passed through Aasimar lands and sometimes even stopped there. This rose suspicion amongst the Aasimar and there was a call for another meeting, on nuetral ground, to sort out what was going on. High Prophet Gieliah, leader of the Thrarian Covenant, took advantage of this situation and set a trap for her own people, framing the Tieflings; luckily for her she had been formally requested not to attend the meeting because of her prejudice. When the trap was sprung a number of important figures amongst the Aasimar lay dead and Gieliah was one of the only Aasimar high nobles left to call upon. Other lesser nobles turned to Gieliah and the Thrarian Covenant for direction and thus began The Quelling.

The Quelling led to the near eradication of the Tieflings due to the scattered forces of the Tiefling Conglomerate after the events of The Year and a Fortnight Skirmish.