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.
It was a quick read through almost a skim. I was grateful to find a copy that hadn't already been written in. Unfortunately I can't say the same for another Merlin annual I just got. I'm glad I have this is what I'm saying.
The Adventures of Merlin: The Official Annual 2013 (2013) is a great annual about the TV show.
The annual has lots of information about the characters in the TV show and the story of the TV series up to that point. The book also has puzzles and quizzes. There is a wonderful two-part story in the annual called The Sword In The Stone.
It is very well put together and informative. A lot is packed into it which would keep fans of Merlin occupied for hours. I think it was very well thought out and a very enjoyable annual.