Glorantha started out with Chaosium. Glorantha's inventor, Greg Stafford, created the company to publish his first Gloranthan boardgame 'White Bear Red Moon' in the early 70s.
RuneQuest was the first RPG for Glorantha, developed towards the end of the 70s. It remained with Chaosium until the mid 80s when Chaosium sub-licenced production to Avalon Hill (whilst still maintaining creative control over most RQ output, and all Gloranthan specific output). The relationshipo was never fabulous but did produce some memorable work from Avalon Hill, including Dorastor and Sun County. But Glorantha's 'golden age' has always been considered as the Chaosium years.
When the relationship with Avalon Hill ended, in the early 90s, they kept the name RuneQuest but rights to Glorantha returned to Chaosium. For reasons too detailed to go into here, Greg set up Issaries to publish a new Gloranthan RPG, which became Hero wars.
The trademark for RQ returned to Greg (although there was, apparently, a rush to claim it by both Chaosium and Issaries) about 3 years ago. Issaries won, and owns the trademarks for RQ and Glorantha.
Glorantha and RQ are separate entities. Mongoose licence RQ from Issaries and part of the deal is to publish material set in the Second Age. Most material published under Issaries and Moon Design (which now licences HeroQuest) is set in the 3rd Age - the 'classic' Gloranthan setting. This allows both Mongoose and Moon/Issaries to publish games set in Glorantha using separate systems.
Chaosium's financial troubles are not down to the Chaosium/Issaries split (although that clearly didn't help). Chaosium invested heavily in the Mythos card game during the CCG boom of the 90s and had a lot of success with it; just prior to Mythos it had suffered because the CCG boom depressed sales of roleplaying games. But when that bubble burst, it hit hard: not just Chaosium but a lot of smaller publishers too. Chaosium's weathered a bad storm but kept going and is on the verge of publishing a new, Deluxe version of the Basic Roleplaying system. Which curiously enough is the grandfather (if not the father) of Mongoose RQ.
Shannon Appel has written a great precis of the Chaosium/Issaries relationship for RPG.net and for a fuller, more accurate version of the story, it's worth a read. Dunno what the link is, but point your browser at www.rpg.net and use the search facility there and you should be able to turn it up.
So yes, Mage. A confusing and detailed saga when examined, but fascinating nonetheless.