From 1d4chan

A goddess in Warhammer Fantasy. Very un-grimdark in that she is the patron god of healing and mercy. Daughter of Morr and Verena, sister of Myrmidia. Her symbol is an eye filled with tears, making an interesting parallel between her and Isha. It's possible that it's simply Isha taking an interest in human affairs, or just a shared domain. Speaking of tears, her statues are said to weep her sacred tears, which have powerful properties. Ranald, God of Thieves and Trickery, is said to have convinced Shallya to let him drink a vial of her tears to gain godhood. In the mortal realms the tears are a powerful healing agent.

Shallya is the enemy of the Chaos God Nurgle, the two gods having opposing realms to each other. Nothing strikes more fear into the hearts of the priests of Shallya then the work of the Plaguefather, and even the most disgusting of Nurgle's minions despair at Shallya's purity. Unfortunately by the events of The End Times, though the Shallyites preach otherwise, Nurgle is the stronger of the two, and has imprisoned her in his pestilent kitchen, forcing her into slavery as the "Poxfulcrum", a distorted daemon upon whom Nurgle tests all of his latest plagues and concoctions before potentially deploying them to the mortal world. Isha takes this role in 40K. Assuming he doesn't have both of them. Papa Nurgle's love is enough for everyone. Ugh.

During the End Times Araloth and Kalara make a dangerous trip into the Realm of Chaos at the behest of Lileath. Kalara takes Shallya's place as the Poxfulcrum to keep Nurgle deceived, whilst Araloth helps Shallya escape, allowing her to aid in curing Taal and thus breaking Nurgle's currently powerful grip on the mortal world. Though she is shown returning to full strength at the end of book 2, by the events of Thanquol she and the other old gods have faded for whatever reason.

The Gods of the Empire
Manann - Morr - Myrmidia - Ranald - Rhya - Shallya - Sigmar - Taal - Ulric - Verena