Nyalee

Nyalee was a priestess of Bhaal who adopted the bhaalspawn fire giant known as Yaga-Shura. By using her magic, Nyalee was able to make Yaga-Shura invulnverable by removing his heart from his body and keeping it in eternal burning flames. As long as the heart stayed there, Yaga-Shura could not be harmed. To make sure Nyalee would not reverse the rites that made him invincible, Yaga-Shura took her heart and stored it in his enclave, thus making it impossbile from her to perform such rites.

In Baldur's Gate II, you will meet Nyalee in the Bhaalist temple in the Forest of Mir. She will tell you to make Yaga-Shura vulnerable you need to bring her both his and her own hearts to her. When you return the hearts to Nyalee will use her powers and make Yaga-Shura mortal again. However, returning her heart has also returned her feelings and emotions, including her love for her adopted son. Knowing her love, Nyalee will then summon many creatures of the glade and attack you with them.