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?
Sentence (from "Order of the Phoenix (fictional organisation)"): The Order of the Phoenix is a secret organisation in the "Harry Potter" series of fiction books written by J. K. Rowling.
Relevant evidence: Yes