What is the name of the killer in 'A Nightmare on Elm Street'?

Which film series is based on the novels of J.R.R. Tolkien? Which film series is based on the novels of C.S. Lewis? What is the name of the killer in the 'Halloween' film series? What is the name of the killer in the 'Friday the 13th' film series? What is the name of the killer doll in 'Child's Play'? What is the name of the killer in 'The Texas Chain Saw Massacre' (1974)? What mask does the killer wear in 'Scream' (1996)? What type of mask does Jason Voorhees famously wear?