Question: when did the battle royale mode arrive in fortnite
[{'answer': 'September 2017', 'provenance': [{'wikipedia_id': '56819881', 'title': 'Fortnite Battle Royale', 'start_paragraph_id': 22, 'start_character': 169, 'end_paragraph_id': 22, 'end_character': 183, 'bleu_score': 1.0, 'section': 'Section::::Development.\n'}]}]