To create a fantasy world (which were) filled with fascinating creatures, Rowling did a lot of research into world mythologies and legends.

위 문장에서 a fantasy world랑 filled 사이에 which were가 생략되어있다고 합니다. filled가 world를 수식해주는 상황인데 왜 which was가 아니라 which were인건가요 ㅠㅠ
설명부탁드려요~ which were 답임은 확실합니다.