Glorantha is the fictional world King of Dragon Pass is set in. The world is named after the great goddess Glorantha that created it. It was divided into two realms, the realm of the gods, and the realm of mortals during the Great Darkness.
Birth of the world
The world is said to have been created by the goddess Glorantha and her Cosmic Court. It was composed of the Elemental gods: Darkness, Water, Earth, Sky and Fire. These original gods were described as "huge" and "distant".
In these times, Time didn't exist, there was no present, past or future, everything happened out of sequence, at the same time.
Smaller gods inhabited the world, the most notable of which was Yelm, the prideful god of the Sun, who ruled over everything with his Fire Tribe.
When the humans came, the god Umath blew Air into the sky, and by doing so, showed Yelm that he wasn't the only one with control over the skies. Ulmath's son, Orlanth, ultimately dethroned Yelm and literally sent him to Hell.
The creation of the world is further detailed in How Things Began.
The Great Darkness
Until the day when the Unholy Trio made their appearance and brought Chaos, and Wakboth the Devil into the world. When Orlanth saw the mess of terror and horror that had taken over the world, he embarked on the Lightbringer's Quest, where, with the help of his friends, he captured Wakboth and created Time. The gods then decided to divide the world into two realms, and minimize their interactions with humans, so that nothing like this would ever happen again.
The two realms
Glorantha, as we experience it in the game, is now divided into two realms, the temporal realm of mortals, and the intemporal realm of the gods.
The gods don't interact directly with the creatures on the other side anymore, but they still are very much present in the Orlanthi's daily life, mostly through powerful blessings. God-talkers can also receive some visions from their gods.
Humans can and do cross into the realm of the gods, provided they have enough magic, in order to reinact their gods' ancient myths.
The Fictional World
The fictional fantasy world of Glorantha was created by Greg Stafford. It is used as a background for a number of Role-Playing games, including RuneQuest, Hero Wars, HeroQuest, and of course, King of Dragon Pass.