Glorantha

The setting of many of the RuneQuest editions (but not all).

The background of Glorantha is split into three Ages (First, Second and Third). Each is generlly supported by a different set of rules, as follows.

The Third Age
The Third Age is supported by the HeroQuest and RuneQuest (1st, 2nd & 3rd editions) rules and supplements

The Second Age
The Second Age is supported by the RuneQuest (MRQ) (sometimes known as the 4th edition) and RuneQuest II (5th edition), both published by Mongoose Publishing.

Players do use the rules system with the background they prefer (mix and match). (Also, some players use the Basic RolePlaying by Chaosium or OpenQuest instead).