Beholders already believe themselves to be superior. Even if each of them is some twisted, horrifying form, they believe that their form is correct, and all the other quirks in the other are lesser. Ixrixs was unique in that she thought otherwise, that the options were infinite, and only a fool would trust the genetic lottery to finding perfection. It was the pride of other beholders that allowed her to move freely and amass her own skill and power, and it was a curious archmage, believing that her unique point of view was humility, that exposed her to a world she wouldn't have seen in the tunnels on her plane. She worked alongside the archmage, who was blinded by his own pride at 'domesticating' a beholder, not to see that she was only playing along in order to learn more.
He succeeded in crafting her a new form, and he was eager to show off his new creation to the world. He called a caucus of other mages from across the planes to see and discuss Ixrixs' new form, but organizing such an event took time. In the months it took, Ixrixis' prowess increased exponentially in her new form, her salvaged beholder traits evolving to something new. Even if the archmage was nervous about her growing greed and power, he couldn't risk cancelling and losing his reputation. Ixrixs' attended the caucus, and was on her best behavior. She was hoping that a meeting of minds such as this would allow new insight, further developments.
The assembled mages had no usable input, or nothing that was usable enough. Ixrixs quickly grew upset, and gave a full demonstration of her power in slaughtering as many of the mages as she could. The same safeguards that were designed to protect the mages from each other had kept them trapped in with her. Just enough escaped to warn the world about what had been created, rumors of her spreading while she explored the tower uninhibited, explored all the information that the other archmages had brought to the caucus. She was about to leave, to track down the others and collect more information, until Ao himself stepped in to answer the calls before she could be a blight. She was imprisoned, and her memory remaining as a threat to any other wizards out there: a boogeyman that would strike any wizards who had too much hubris.