God Time: In the God Time, there was no division between the worlds of men and gods, between Life and Death, or body and spirit. The events and deeds of the God Time made the cosmos and established its patterns.The Gods War introduced the power of Death, which separated all with violence, death, and entropy. Reality was fractured. The eternal gods would have ceased to exist but for the Cosmic Compromise, whereby the gods voluntarily abdicated their free will within the temporal world rather than lose all into the maws of nonexistence. The gods remained eternal at the cost of being limited to their God Time patterns.
Hero Plane: This plane exists simultaneously in the temporal Mundane World and the eternal Gods World (and sometimes the Underworld). There live legendary beings, including worshiped heroes or traditional enemies. Incarnations of the gods manifest easily here and interact with beings and things. There are landscapes, peoples and individuals, and the usual physical laws of the Mundane World mostly hold true, thus people can walk about, eat, talk, and otherwise physically interact with the world. However, both Time and the God Time interact here, as a result an adventurer in the Hero Plane may simultaneously interact with gods (or at least their incarnations) and with purely mundane things.
Gods World: The realm of eternity. Every god and goddess who performed great immortal deeds is found here. Within this world of extremes lie the heavens and hells of the cults, where initiates and devotees go after death. Here, too, lie the great pools of creative material from which were made the primal oceans, ancient mountains, and first skies of the mundane world.
Mundane World: This is the world of mortal beings, bound by Time. This is the physical world, of Life and Death. This is the world of your RuneQuest adventurers.
Spirit World: This world is made up of non-physical (discorporate) entities and things. It has a correlation with the Middle World in that many spirits maintain their relative spatial positions in the Middle World and the Spirit World. It is a place of sublime beauty and life, but also where lost spirits wander, where broken dreams coalesce, and where nightmares feast.The Spirit World can be visualized as a place without gravity or an exterior light source, but which is instead filled with various shapes of color. Real perception is limited, and depends on both the power of the viewer and the viewed.The Spirit World is made of many smaller parts that are distinct but connected to other locations through spiritual pathways.
Underworld: The Underworld is the irrational ancestress of known Glorantha. It precedes all that can be known, and thus cannot be truly known. It houses mystery, spawns mystery and is a mystery itself. Many Gloranthans think that all the worlds of Glorantha spawned from the Underworld.
Hidden deep underneath the center of the bottom of the lowest Underworld is the Chaosium, the Fount of Chaos, which spews forth both monstrosities and raw unformed “stuff” into the world. What no sane Gloranthan really understands is that, without this Chaos entering the world, everything would be locked into Stasis.
The Compromise in a nutshell: the Compromise is Time. Time includes a certain amount of Chaos – entropy, degeneration, etc., all are features of Time. But Time also has laws, which if broken would allow Chaos to reenter the world and destroy it.
Note that Mundane World, Middle World, world of Time, etc. are referring to “our” world.