When Xi Wan made the trade with the Wing Tiger Tribe Leader, he never told him that Su Yu had a physical defect. He was afraid the leader would find her undesirable and refuse the deal, leaving Xi Wan stuck with her.
So he concocted a ridiculous story, telling the Wing Tiger Tribe Leader that he had rescued Su Yu from the hands of Stray Beasts.
He claimed Stray Beasts had killed her two Beast Husbands and razed her tribe, and that she had cried herself unconscious from grief.
Before she passed out, she had supposedly asked Xi Wan to find her another Beast Husband.
After much consideration, Xi Wan said, he brought her to the mighty Wing Tiger Tribe.
The Wing Tiger Tribe Leader, thinking of his eldest son who was frequently losing control due to an excess of impurities in his body, had made the deal with the treacherous merchant. That was why he'd kept Su Yu and sent her to Xun Yu's stone house.
After all, before he started losing control, Xun Yu had been a very charismatic male.
