Complex sentence: Not one but two Gravelings were present, but, for reasons not yet explained, they let her live, and overall, the series has implied that she has had more than one Graveling sighting as a child.
Split into simpler sentences: Not one but two Gravelings were present, but, for reasons not yet explained, they let her live. The series has implied that she has had more than one Graveling sighting as a child.