In the pre-history of Glorantha, the gods created the world and gave it life. Then they fell to contention: Orlanth slew Yelm the Sun, and when Yelm arrived in Hell, his light and heat drove many trolls and horrors to the darkened surface. Also, The Unholy Trio broke the foundation of the world and let Chaos in to tear Glorantha apart. When all seemed hopeless, the gods joined in the Great Compromise with the mysterious Arachne Solara to wrap up the crumbling remnants of the world in her web. Arachne Solara gave birth to Time, which separates the world of mortals from that of the gods.
- from Hero Paths
, by Kallandri Longfeather, "the Iron Librarian"
The Great (or Cosmic) Compromise limits the ways in which Glorantha's gods can intervene in mortal affairs. It also creates a weird discontinuity in history at the point where Time was born. To mortals, events before the Great Compromise appear to have all happened concurrently, although there are recognizable chains of cause and effect in the popular myths. Finally, since the gods can only affect conditions in Time through their worshippers, the gods may be perceived as quite "static" or "passive" from the mortal perspective. This latter condition is where the gravest cosmic danger lies.
While the gods are constrained to intervene in the mortal plane through their worshippers, mortals are NOT limited to the mundane world. Powerful mortals may venture beyond Glorantha's mundane plane into the Hero Plane, or even into the God Plane. In the Hero Plane, they may travel alien worlds, and often their interactions with parallel cults will reward them with powers and magical treasures. In the God Plane, they are interacting directly with the mythology of their own world, outside and before Time.
At first, mortals were satisfied with quests to reenact the deeds of the gods, perhaps gaining exceptional powers or securing prosperity for their communities that way. For example, a troll hero might quest to the Hill of Gold where the troll god Zorak Zoran beat up Yelmalio and took his fire powers. Conversely, a Yelmalio cultist might undertake the same quest, but to withstand the battle and bring fire magic to his community. Mortals soon came to understand that there are many paths in the Hero Plane where Glorantha's gods had never walked, where a bold quest might return a hero to Glorantha with powers outside the purview of the gods.
Inevitably, some mortals made a science of manipulating the gods and charting the magical planes. In Glorantha's First Age, the first major use (or abuse) of this "technology" resulted in the creation of a new god, variously known as Osentalka, Bright Nysalor, or, to his detractor Arkat, Gbaji the Deceiver or Wakboth the Devil.
Arkat is a complicated figure in Glorantha's First Age. He started out as a warrior of the Brithini people, a monotheist (or perhaps atheist) caste-structured culture, but his career included stints as a Storm cultist, an incarnation as a troll (and Zorak Zoran cultist), and long periods when he was questing for powers to fight Gbaji. Depending who you ask, he saw the truth of the gods' supremacy and piously smote Gbaji on their behalf, or he used the powers of the gods against them to help free mortals, or he was himself a devil who cut short Nysalor's golden age, or he and Nysalor were both evil powers whose struggles merely wasted the world for mortals. (One of the great things about a Glorantha-based game is there are so many perspectives to role-play from!)
Because of disruption wrought by the metaphysical recklessness of later ages (and the intentionally flexible history of Glorantha as a game world), there are many mysteries to Arkat. Several notes do remain consistent, however: Arkat is known as "the father of intentional heroquesting", he was initiated in the mysteries of Illumination that also characterized Nysalor's cult, and he adopted and abandoned (some say "betrayed") multiple cults and communities in his career, before finally defeating Nysalor. Saint or apostate? I think everything necessary to form an opinion on Arkat is contained within his injunction to would-be heroes: No HeroQuesting without humility.
Sadly, Arkat's commandment wasn't heeded for long: the defeat of Nysalor ushered in Glorantha's second age, the Age of Empires. These (predominantly human) empires were soon fueled once again by reckless meddling in the Hero and God planes, and new heights in magical excess. Sorcerors of the Jrusteli ("Godlearner") empire, in particular, killed, bound, and deceived various gods and twisted the normal cycles of nature so dramatically that Glorantha and her dragons (divine powers in their own right) revolted under the strain. The Jrusteli homeland was swept under the waves, and dragons laid waste to much of human civilization. When mortals again sought the hero paths, they found that many of the old ways were barred or trapped by agents unknown.