Kassia was a Consul of Traken. Unlike her fellow Consul and husband, Tremas, she was a "gifted sensitive" and so represented a different constituency than him. Since her youth, she had been assigned by the Keeper to tend to a Melkur in the Grove. Over time, she became infatuated with it.
As a part of her wedding blessing, the Keeper decreed her new stepdaughter, Nyssa, should take over the duties. Because of her strong connection to the Melkur, however, Kassia continued to seek the Melkur's company. During one such visitation, the Melkur began to speak to Kassia. Unbeknownst to her, the "Melkur" was in fact the Master.
The Master used her feelings toward the Melkur for his own purposes. He used her to orchestrate an elaborate coup. At the insistence of what she believed the Melkur, she killed and manipulated her way into the Keepership itself — even though her actions estranged her from her husband, the rightful heir to the Keepership.