As Chaosium moves forward with its new version of RuneQuest, The Design Mechanism had to make a decision as to what to do with its rule-set, which could no longer use the name. Rather than abandon the decades of work (as they had originally worked on RuneQuest II and Legend) they had done, they decided to re-name their game as well as create an introductory booklet that focused on more than fantasy, named Mythras Imperative.
In addition to the confusing name (if Mythras itself will focus more on fantasy, as far as I know), I’m not sure how The Design Mechanism is able to release their games, as they no longer have a license from Chaosium nor are they basing their rules on the OGL’ed Mongoose rules. While they claim to have their own license (but no details are in the booklet nor on their site), it’s certainly not as open as Mongoose’s Legend or D101 Games’ OpenQuest. I have no doubt Mythras will continue to be a high quality of work, but also a high complexity of rules as well as page-count.