Complex sentence: While Drogon's whereabouts are unclear, as a precaution, she locks and chains her two other dragons away in the catacombs, which breaks her heart as she was somewhat of a mother to them.
Split into simpler sentences: While Drogon's whereabouts are unclear, as a precaution, she locks and chains her two other dragons away in the catacombs. This deeply breaks her heart, as she was somewhat of a mother to them.