The first level editors appeared in early 1994, and additional tools have been created that allow most aspects of the game to be edited. Although the majority of WADs contain one or several custom levels mostly in the style of the original game, others implement new monsters and other resources, and heavily alter the gameplay. Several popular movies, television series, other video games and other brands from popular culture have been turned into "Doom" WADs by fans, including "Aliens", "Star Wars", "The Simpsons", "South Park", "Sailor Moon", "Dragon Ball Z", "Pokémon", "Beavis and Butt-head", "Batman", and "Sonic the Hedgehog". Some works, like the "Theme Doom Patch", combined enemies from several films, such as "Aliens", "Predator", and "The Terminator". Some add-on files were also made that changed the sounds made by the various characters and weapons.