Complex sentence: Tywin discovered that the young Tyrion eloped with a peasant's daughter and fell into a cold rage, believing that a Lannister should never stoop so low as to marry lowborns.
Split into simpler sentences: Tywin discovered that the young Tyrion had eloped with a peasant's daughter. The patriarch fell into a cold rage, believing that a Lannister should never stoop so low as to marry lowborns.