Grimm is a dark world of fairy tales gone wrong. This second book in the Horizon line of d20 mini-games puts the players in the role of children trapped in a nightmare world. Its residents are the familiar characters of childhood nursery rhymes and fairy tales, but few creatures here are friendly, none are harmless, and more than a few want to find out what little girls and boys are really made of...
Various is the correct author for any book with multiple unknown authors, and is acceptable for books with multiple known authors, especially if not all are known or the list is very long (over 50).
If an editor is known, however, Various is not necessary. List the name of the editor as the primary author (with role "editor"). Contributing authors' names follow it.
Note: WorldCat is an excellent resource for finding author information and contents of anthologies.