Lord Ahriman spookily makes his debut in the feast after The Battle of King's Landing, preaching about the matters of life and death to King Aeyron, asking him for the body of Ser Gregor Hull 'The Mountain'.
-As far as I remember he doesn't reappear in any chapters so it is unclear whether he got The Mountain's body or not but his ambitions with The Mountain's body are not told. We have seen zombies in the series-with The Ghost of Blackfyre's skeletons that were brought back to life but she were a powerful mage and Ahriman is just a Lord of the Pyromancers.
-Linking to the Undead Ulthor Theory, perhaps Lord Ahriman will attempt to bring Ulthor back to life, or even worse maybe Aeyron will demand Ahriman to bring Joanna to life after he successfully returns The Mountain to the realm of the living.