Complex sentence: Thomas Swynford, a son from Katherine's first marriage to Sir Hugh Swynford was another loyal companion and Constable of Pontefract Castle, where King Richard II is said to have died.
Split into simpler sentences: Thomas Swynford, a son from Katherine's first marriage to Sir Hugh Swynford, was another loyal companion. Thomas was Constable of Pontefract Castle, where King Richard II is said to have died.