Question: The fictional character in J. K. Rowling's "Harry Potter" series Severus Snape becomes a double-agent on behalf of this secret organisation to fight Voldemort and his followers known as what?
{'answer': 'Death Eaters', 'supporting_facts': [['Severus Snape', 0], ['Severus Snape', 4], ['Order of the Phoenix (fictional organisation)', 0], ['Order of the Phoenix (fictional organisation)', 1]]}