Yagna was one of the orphans that Gran looked after in Crookback Bog in 1272.
If Geralt freed the Ghost in the Tree: Yagna, along with all the other orphans from Crookback Bog, was saved by the Ghost from the Crones. The orphans later ended up in Novigrad, and were placed under the care of Marabella at her school.
If Geralt killed the Ghost in the Tree: Yagna, along with all the other orphans from Crookback Bog, was taken to Aard Cerbin by the Crones, where she was killed and consumed by them.