Isha is the daughter of Asuryan and Lileath. She is a merciful god who sends aid to those most in need of it. It is her blessing that keeps the province of Avelorn free from winter's touch. Isha is commonly portrayed as a gloriously beautiful elf maiden full of life, with long golden tresses flowing around her. Her symbol is an eye shedding a tear, which represents the tears Isha sheds for her mortal children.
In Warhammer 40,000
Long before the Fall of the Eldar, Khaine, the Eldar god of war, learned of a prophecy, which stated that an Eldar would kill him. As a response, Khaine started a systematic purge of the Eldar. Isha turned to Asuryan, the king of the Eldar pantheon, pleading with him to stop Khaine. At first, Asuryan dismissed her request. However, he would eventually order all the Gods to cease any and all interactions with the Eldar. Isha met with the Eldar in secret, but was imprisoned, along with her husband Kurnous, when Khaine found out.
The other gods found this to be extreme, but did nothing to help her. The exception to this was the smith god, Vaul, who offered to make one hundred swords for Khaine in exchange for the couple's release. Khaine agreed to release them in a year if Vaul could complete the blades. However, by the time his year was up, Vaul had only finished 99 swords. Instead of leave Isha and Kurnous to rot in Khaine's dungeon, he replaced the 100th sword with a regular one. Khaine was tricked at the time and allowed the couple to leave. However, when he found out, he swore revenge against Vaul, sparking what would become known as the War in Heaven: a cataclysmic battle between Vaul and Khaine, with other gods taking sides, whilst a similar battle raged amongst the mortal Eldar. The battle between Vaul and Khaine ended when Khaine struck Vaul down, imprisoning him in his own anvil, whilst the War in Heaven ended with the death of Eldanesh, at the time the most powerful Eldar leader, at the hands of Khaine.
During the Fall, most of the Eldar gods were killed. However, Isha survived Slaanesh's slaughter of the other Eldar gods. Her fate was instead to become Slaanesh's prisoner. None can say what Slaanesh's intentions were, although it can be assumed that given Slaanesh's title of "Prince of Ecstacy," death would've been a better alternative. Before she could be returned to Slaanesh's palace, however, she was saved by the Chaos god of disease, Nurgle, who waged his own war against the newly-born god, presumably to put Slaanesh in its place. Nurgle's disease-ridden host defeated the forces of Slaanesh and the Lord of Decay claimed his newborn brother's prize as his own.
Nurgle came to adore his new companion like no other being in the universe, and constantly shows his love in the only way he knows how. Nurgle keeps Isha imprisoned in a rusted cage within his personal manse in the corner of his Plague Gardens. It is there that he keeps the cauldron where he mixes the elements that create all of his plagues and pestilences. Nurgle uses Isha as a test subject for his latest poxes he plans to release on the universe.
As the Goddess of Healing, Isha can cure herself of these afflictions, but Nurgle gains an idea of their potency by watching how much power she expends to cure herself. If he is pleased at how hard Isha must struggle to conquer the disease, Nurgle releases the new creation upon some unsuspecting world, but if not, he starts over, working at his cauldron until he has something new to give to his companion. Isha accepts her lot stoically, whilst whispering the cures to these new diseases into the universe, allowing mortals some chance at resisting Nurgle.
It is believed that the Eldar Spirit Stones are the crystalized tears of Isha.