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Here's what happened:
After the events of the last expansion for Heroes of Might and Magic V, he escapes from Isabel and Illia, who want him to save their kingdoms. He returns to Shi'o, where he quietly overthrows Kha-Beleth and takes his place. There, he meets Biara, who turns out to be alive. Tired of Isabel's constant nagging, Agrael agrees to Biara as his second wife, and they have a succubus daughter named Xana, who has the same beautiful black hair as her father. In short, she inherits the best traits from both parents.
Around the same time, the cunning Agrael has a child with Isabel, named Liana. And then, the son of Isabel and Kha-Beleth accidentally locks Agrael, who is now in Kha-Beleth's place, in Shi'o... and this is what sets the whole plot of Dark Messiah in motion!