Complex sentence: In the third quatrain, he talks about his fears of death robbing him of his love, the "fair creature of an hour," which is believed to have been his fiancé, whom he was secretly engaged to.
Split into simpler sentences: In the third quatrain, he talks about his fears of death robbing him of his love, the "fair creature of an hour". This is believed to have been Keats' fiancé, whom he was secretly engaged to.