For Chaosium Games, Runequest became a cornerstone of their publishing. Their tabletop RPGs took ideas or the system from Runequest and adapted it to things like Lovecraftian horror and Occult Fantasy (if you've never seen Chaosium's translation of the French RPG Nephlim, you need to find a copy of it). Along the way Chaosium acquired the license to Michael Moorcock's Elric Saga and published boardgames and a series of role-playing games based on Moorcock's characters and worlds. This lead to one of the earliest dark fantasy games, based on Moorcock's Elric called Stormbringer (with a brief sidetrack into the Elric! game for its 4th edition). Stormbringer spawned five editions and a wide variety of supplements over the course of its history.
Flash forward to today (or a few months ago when this book came out) and Chaosium found itself in a hobby where fantasy is king, and they were no longer able to publish either of their fantasy properties. No more Stormbringer because the license to Moorcock's stuff is with another publisher, and no more Runequest name because, well, a lot of stuff happened. What's a publisher to do when they have the system and no name to go with it? Simple, they dig around in their history for a name that they do own (Magic World from the old Worlds of Wonder boxed set) and build a new game around it.
Magic World is literally built upon the shoulders of giants. Editor Ben Monroe went through years worth of Stormbringer material and carefully weeded through it to take out the references to Moorcock's work but kept all of the flavor and assumptions of the underlying system's approach to fantasy gaming.
The system eschews the standards of class and level for a skill-based approach that puts more importance on what a character knows and is capable of doing, and letting that define the character. While using the same Basic Roleplaying System that was originally developed for Runequest, it is a standalone game and does not require anything else to play. You could use the big gold book, Basic Roleplaying, from Chaosium to give your Magic World game more options, but it is not a requirement. And if you're looking for creatures to populate your world, everything from old Runequest supplements to the Malleus Monsromo (one of the best monster manuals ever made) for Call of Cthulhu to the old Stormbringer stuff will be compatible. Some material, like the Call of Cthulhu monsters, may require a minimal bit of adaptation, but ultimately it will all fit. By its nature and ancestry, Magic World has a lustier approach to fantasy than with many other fantasy RPGs. For me, this is a good thing because I think that Magic World has less of a high fantasy feel to it (as typified by Tolkien and his many, many imitators) and more of (what I hate to call) "low fantasy" approach. I would use the label Swords & Sorcery, but I think that this would pigeonhole the game and make people think that it is capable only of characters like Elric or Conan or the Grey Mouser.
Magic World is available in PDF and print, either directly from Chaosium Games, or via traditional distribution through your local gaming store. If you want fantasy, but you are looking for something that is different from many of the fantasy games that are out there, you would not do wrong to check out Magic World.