Complex sentence: In 2013, he was cast in an unknown role in the HBO series "Game of Thrones", though it was previously thought to have been playing Styr, the Magnar of Thenn.
Split into simpler sentences: In 2013, he landed a role on the popular HBO series "Game of Thrones". Gatt will play a yet unknown character, but was previously thought to have been playing Styr, the Magnar of Thenn.