You're asking about Isha.
Isha is the Eldar Goddess of the Harvest, Fertility, Life, Healing and Growth and the mother of the Eldar race in Eldar Mythology. She is the mate of the god Kurnous, the God of the Hunt. The only truly benevolent member of the Eldar Pantheon, it is said that it was Isha who intervened and created all non-Eldar life in the universe once Asuryan, the ruler of the Eldar Pantheon, no longer took interest in his creations and their solitude. It was Isha who pleaded on behalf of the mortals when the Eldar God of War, Kaela Mensha Khaine, attempted to slaughter the Eldar, having heard from the Crone Goddess Morai-Heg that the Eldar would one day cause his downfall, and it was Isha who secretly continued to guide the Eldar when Asuryan interdicted any direct contact between mortals and Gods by separating the Immaterium from the Materium.
After the Fall of the Eldar, Isha was about to be devoured by Slaanesh like all of her divine brethren, but she was rescued by the forces of the Plague God Nurgle. Today, many Eldar believe that Nurgle keeps Isha in a cage in his Plague Gardens in the Realm of Chaos, where he tests out every new disease and pestilence he creates to affect the mortal realm. But Isha still seeks to aid mortals by whispering the cures for each of Nurgle's plagues into the universe to be discovered when they are most needed.
She apparently is referenced in the Warhammer 40,000: Compilation (1st Edition).
At the very least, the Descendents of Isha are units in WH40k and she exists in legend.
Since the formation of the Great Rift, rumors spread among the Exodites that the Masque of the Frozen Stars seeks to free Isha from Nurgle's grip. It is whispered that if enough Eldar weep for the corruption of their Maiden Worlds, their combined sorrow could release Isha from her prison.
She exists in Warhammer Fantasy as well, Warhammer Fantasy RPG 2nd ED -- Tome of Salvation and Warhammer Armies: High Elves (8th Edition), but the incident you refer to is part of the 40k lore.