Isha is the Eldar Goddess of healing, fertility, and the harvest.
Official Canon Edit
Isha is considered the mother of the Eldar, and has a close bond with her children, speaking to them through her spirit stones. After the War in Heaven, when one of the other gods predicted that the Eldar would kill Khaine, he decided that he would kill every Eldar to keep the prophecy from coming true. Asuryan stayed his hand by requiring all Eldar gods to remain separate from the Eldar. Isha however stayed in contact with them in secret; when Khaine discovered this, he attempted to claim Isha as his slave until Vaul intervened.
Later, when Slaanesh was born, the new Chaos God consumed all of the Eldar Gods save for Cegorach, Khaine, and Isha. Isha cried out, which was heard by Nurgle. Nurgle intervened and claimed Isha for his own, and she has since been trapped in his realm. He forces Isha to try his new diseases, which she is able to heal from.