Becoming the vampire spawn and giving up your freedom for love
As I said, all Ascended Astarion content will be posted in a specific playlist and is not Damenys canonical. This scene is Astarion's romantic scene from act 3 which changes whether he ascends or not.
In this timeline Damenys gives up her mortality and agrees to become the offspring of her beloved Astarion so that they can live out their love for eternity.