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 for her to perform such rites.
In Baldur's Gate II: Throne of Bhaal, Gorion's Ward and their party will meet Nyalee in the temple of Bhaal in the Forest of Mir. She will tell them that to make Yaga-Shura vulnerable, they need to bring her both the giant's and her own heart to her. When the hearts are returned to Nyalee, she 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. Remembering her love, Nyalee will then summon many creatures of the glade and attack.