Complex sentence: Cardinal Demetrius, who had accompanied Hedwig to Poland, remained abstent from the queens' court after his return to Hungary, thus obstructing the functioning of royal government because he was the keeper of the royal seal.
Split into simpler sentences: Cardinal Demetrius, who had accompanied Hedwig to Poland, remained abstent from the queens' court after his return to Hungary. The royal government could not properly function during his absent, because he was the keeper of the royal seal.