Along with her colleagues in the Lodge of Sorceresses, she made a conspiracy against several kings of the Northern Realms. The sorceresses plotted to unify the kingdoms under a puppet ruler so they could control the North. Sheala herself is not perticularly evil and distains politics or national involevement, yet she did play a heavy hand in the plot as she saw a need to.
She hired Letho of Gulet to assassinate King Demavend, but after he killed Foltest, she figured out he works for Emperor of Nilfgaard and tried to find him in order to destroy all evidences of her involvement in this regicide. Unfortunetly, Letho know more than she bargained for and would try to be her undoing...
She protected King Henselt of Keadwen from Letho's colleagues, but Geralt of Rivia eventually discovered she was Letho's client and former lover. She would prove to be evasive and more of a conflcted character than a real villain. In short she was a loner and best described as one obsessed with privacy and personal research, too bad she got involved in the Assassination of the Northern Kings.
She followed the orders of Philippa Eilhart to control her dragon. Depending on player's choices she can die or escape.
If Geralt saves Sheala then he and Yennefer later find her in a prison in Oxenfurt when they are looking for Margarita Laux-Antille, who got captured by King Radovid. Sheala has been extensively tortured beyond the possibility of being saved (as Letho previously mentioned, something offal) so either Geralt or Yennefer put her out of her misery in order to spare her from a far more agonizing death.
The Witcher 2: Assassin of Kings
The Witcher 3: Wild Hunt