Complex sentence: There was a famous anecdote that during Bidam's rebellion, Queen Seondeok's illness had gotten worse and a star fell, which Bidam used as a sign of the end of the Queen's reign, to encourage his followers.
Split into simpler sentences: There was a famous anecdote that during Bidam's rebellion, Queen Seondeok's illness has gotten worse and a star fell. Bidam, used it as a sign of the Queen's downfall to encourage his followers.