She is the mother of the Eldar race and it was she who inspired the creation of Asuryan's barrier between the mortals and their gods, weeping over the destruction that Khaine was causing. It is also said that the spirit stones are made out of her tears. The spirit stones allowed the gods and mortals to communicate with each other. One of these stones was given to Isha and the rest to the Eldar. When Asuryan learned that his order had been violated he gave Isha and her lover, Kurnous, to Khaine to do with as he wished. Khaine tortured the two of them in a burning pit until Vaul, the only Eldar god who was moved to, struck a bargain with the war god to create one hundred swords for him in exchange for their release. Vaul failed in his task but tricked Khaine and when Khaine found out it sparked off a war between him and Vaul.
Millennia later, the Fall of the Eldar took place and all the Eldar gods except Khaine and Cegorach were believed destroyed by Slaanesh. However, there is a single Craftworld which tells the tale of Isha surviving her fate, being claimed by Slaanesh as his. She cried out for help and was heard by Nurgle who entered into a long war with his newest peer. Nurgle emerged victorious and took Isha as his companion. A goddess of rejuvenation and a god of decay seemed an odd pairing, but Nurgle adores her like no other. However, Nurgle shows his adoration as only a Chaos God can, keeping her trapped in a cage in the Garden of Nurgle, in the corner of a room where he keeps the cauldron in which he creates all of his plagues.
Being a goddess of healing, Isha can cure herself of any of Nurgle's diseases. Nurgle takes advantage of this by force-feeding her his latest creation and sees how long it takes the goddess to overcome its effects. If he is pleased, he releases it upon some unsuspecting world, if not, he starts over, working at his cauldron until he has something new to give to his "companion." Whilst he is busy working though, Isha takes advantage of his distraction to instruct mortals on how to rid themselves of Nurgle's poxes.