Chaosium is one of the longer lived publishers of role-playing games still in existence. Originally founded by Greg Stafford, its first game was actually a wargame, White Bear and Red Moon, which later mutated into Dragon Pass and its sequel, Nomad Gods. White Bear and Red Moon is notable for containing the first published material about Glorantha, later immortalized as the primary setting for the role-playing game RuneQuest and, now, Hero Wars.
The rules of RuneQuest, Chaosium's first role-playing game, were distilled down into a generic, genre-agnostic format known as Basic Role-Playing (BRP). These generic rules formed the basis of many, if not most, of Chaosium's later RPGs, such as Call of Cthulhu, Stormbringer, Nephilim, and Ringworld.
Chaosium and Greg Stafford are also responsible for Pendragon, an Arthurian RPG. Other games of note include Mythos, Elfquest, Worlds of Wonder, Superworld, Hawkmoon, and the frequently forgotten Prince Valiant.
Some Chaosium products have been translated into French, German, Spanish and Italian, and were available in France from Jeux Descartes, in Germany from Pegasus Press, in Spain from La factoría de ideas and in Italy from Stratelibri and Grifo Edizioni. Call of Cthulhu's Quickstart Rules were translated to Hebrew and distributed free by The Orc Magazine.
In the mid-1980s, Chaosium entered into a complex arrangement with Avalon Hill to publish RuneQuest material while Chaosium maintained editorial control over Glorantha-based material for the game (which Avalon Hill would publish). While this agreement reportedly kept the company in existence, it also left RuneQuest moribund, leading to products of questionable quality, long gaps with no products published at all, and, eventually, the death of the game altogether.
In the late 1990s, Chaosium effectively split up into various successor companies, each maintaining its focus on a few of the company's products. Green Knight Publishing formed to focus on Pendragon, Chaosium "proper" retained Call of Cthulhu, Stormbringer, and Mythos, while Greg Stafford founded Issaries to publish Hero Wars and focus on bringing new Glorantha related material into print. Also, Wizard's Attic was formed in order to act as a fulfillment house.
Original Wiki source: Wikipedia