Sellen is openly against the Carian royals. After defeating Jerren, she claims that Queen Renalla is no more, yet when you rest at the grace, Renalla is back where she was. We can assume, then, that Renalla gave Sellen (and possibly Lusat and Azur, since they are missing from their original locations after the fact) a “rebirth” as punishment for defiance. That’s my take, anyways.
All the replies are wrong. Sellen purposefully turned herself into the ball. The balls are created by fusing multiple sorcerers together. She wanted to become a ball so that she would have enough intelligence attribute to study the primeval current and become a star/god. She has killed many of her apprentices and students by turning them into balls. She even threatens to kill you and turn you into a ball. She says "if you fail to become elden lord, do not worry. I have use for even the dullest apprentices."
