Veteran Actor Ewan Mitchell (Osferth) from The Last Kingdom played Aemond as an adult in the final few episodes of season 1 and will continue to do so for the remainder of the show.
Childhood bullying caused Aemond to develop into a subtly menacing adult. The "oldest, grumpiest, biggest dragon" in Westeros, Vhagar, was claimed by him, changing the course of his trip.
The series is based on George R.R. Martin’s book, Fire and Blood, which tells the story of House Targaryen’s rise to power in Westeros.