Complex sentence: Their forbidden love sparked the quest to wrest a Silmaril from Morgoth, and although they succeeded with great suffering, the great Wolf Carcharoth was also able to breach the Girdle through the power of the Silmaril he had swallowed.
Split into simpler sentences: Their forbidden love sparked the quest to steal a Silmaril from Morgoth, in which they succeeded, but through great suffering. At the end the quest, the great Wolf, Carcharoth, also breached the Girdle due to the power of the Silmaril he had swallowed.