No one knows what came before the gods or where the gods even came from, but the gods are said to be the first beings to harness the primordial forces of the multiverse into something tangible. Genesis and Gruumsh were the first two gods to begin the shaping of worlds. It began as a competition between the two, with Genesis building entire worlds, planes of existence, just for Gruumsh to destroy them and start the process anew. Other gods joined Genesis in creating, adding more complexity to the worlds. Harmonia infused the multiverse with some of her raw magical power, and Terra created the first non-divine beings. These beings are incomprehensible to people now, their primitiveness and alienness would break the mind of anyone today. The gods became especially focused on the material plane, which they all agreed was common ground for experimentation. This world became so complex that Gruumsh was no longer able to destroy it completely, which he soon gave up trying to do and began plotting something else.
Valos and Artanor couldn't agree on whether the world should be bathed in light or shrouded in darkness, so they compromised and agreed that the world would alternate between darkness and light. Artanor got lonely in the dark though, so he asked his wife Naivara to accompany the darkness whenever it spread across the world, thus why the moon always accompanies the night.
Eventually the gods got bored of messing around with the material plane, leaving it in a state that is believed to be similar to the earliest tales tell. They began creating their own domains far removed from the material plane (the outer planes).
The primitive material plane was pretty boring. All of the creatures just sort of existed without really doing much. Being mostly powerless, these creatures were bent to the will of the gods. Harmonia was not a fan of this, and decided to do something to change it. Harmonia sacrificed a part of her very essence in an event now known as the Fracture, which in turn diffused throughout the multiverse, creating a well of raw magical power that could be drawn on by other creatures. Dragons were the first creatures to gain a sufficient mastery of drawing on the well of raw magic. However, it wouldn't be until the Great Dissension that the idea of the weave was created and humanoids learned to draw from its power.
Most of the gods weren't happy that others were able to tap into magical power, as it could be used to rebel against the gods. Even so, Harmonia's sacrifice was irreversible, and the gods were unable to get rid of the magic in the multiverse.
As a consequence, Harmonia was imprisoned in Carceri for 10,000 years.
After the Fracture, the Gods became very concerned about the non-divine beings rising up against them. Mortessa put forth the idea that all non-divine beings should be destined to die eventually, with their physical body being left behind and their soul departing to live out its days on one of the outer planes, usually one that follows the creature's alignment. The idea of death was very foreign to the gods, as they were immortal and couldn't fathom the idea of ceasing to exist. Mortessa took it upon herself to be the custodian of death, making sure that all creatures were destined to die at some point. A creature's death from old age was determined when they were born, but their life could be cut short, and Mortessa would come to determine where their soul was bound to go. On very rare and sometimes legendary occasions, Mortessa decides that one's soul is not yet ready to depart for the outer planes, and spares beings from death in an act of mercy.
Once the multiverse had become so complex that Gruumsh could no longer destroy it, he got increasingly jealous and bored. He and his son, The Blood Chief, decided to enlist Chaos to help them incite madness across the realms and spread death. They went throughout the material plane, slaughtering people and holding onto their souls, preventing them from departing to the outer planes. They put these captured souls into their own vessels, called Orcs, who were crafted to their liking. Once a soul was put into the Orc, Chaos imbued it with a sense of madness, giving all Orcs an ever-hungry bloodlust. Eventually, the gods of the good planes were able to muster enough power to halt their march across the material plane. By this point however, Gruumsh, The Blood Chief, and Chaos had amassed enough souls that they had a very influential bargaining chip, so the gods made a deal: Gruumsh and the Blood Chief were allowed to create their own plane, Acheron, far away from the domain of the existing planes. In return, Gruumsh and the Blood Chief released the souls they had collected to the outer planes. Because Acheron exists outside of the domain of Mortessa, endless battle ensues, with the slain indefinitely being resurrected to fight again.
Chaos however, didn't want to go to Acheron, and as part of the ultimatum was instead banished to Carceri.
Chaos was imprisoned on Carceri as a consequence of the First Blood War. It was during this banishment that The Blood Chief tasked Chaos with assisting in the creation of many of the demons that inhabit The Abyss. The Blood Chief told him that only once he had created an army to rival his father's on Acheron would he be set free. Chaos, a primordial himself, didn't like being commanded by the son of another, and he set about escaping Limbo. No one knows how he did it, but it is said that one day, he simply vanished from the plane. Some say that the passage that he used can still be used by those stuck on the plane today. After escaping Carceri, Chaos found himself without a true domain of his own. It was not his nature to rule over anything with any sort of order. Chaos traveled to the edge of the multiverse, exploring the depths of the Astral Plane. This is known as the Voyage of Chaos. He traveled far beyond the known bounds of the Astral Plane, reaching an area where there was no sense of order or cohesiveness at all. It was perfect. Here, leftover material from the creation of other planes, elemental debris, and random force all collided in a grand cacophony. Chaos wanted to savor this unruliness, so he set about containing it for himself. He used his primordial power to fashion borders to a large expanse of the nothingness. Eventually, this area became separated enough from the Astral Plane that it became its own Outer Plane. This is how Limbo came to be, with Chaos being the lord over it.
After the First Blood War, and the concessions that were made because of it, The Blood Chief was one of the most powerful deities in the multiverse. He wasn't a primordial, so he didn't possess as much inherent power as his father, Gruumsh, but he had garnered just as much respect and reverence. He spent some time ruling over Acheron with his father, but one can only watch goblins and orcs slaughter each other for so long, and he also wanted to make an even greater name for himself. He began by creating an army under his father's nose on Acheron. It was in the creation of this army, that The Blood Chief first began experimenting with creating his own sentient creatures. Said to be the very first demons, these creatures were mixed in with the usual inhabitants of Acheron to create a formidable army. The Blood Chief's army marched on Malus, Gruumsh's Palace, in a show of power against Gruumsh. However, Gruumsh's honor guard made quick work of this army in a brutal slaughter known as the Bloodbath of Malus. Gruumsh was emotionally devastated by his son's treachery. Gruumsh, the malicious tyrant that he is, saw simple banishment unfit for his son, so he cast him and all of his cruel creations into The Abyss. This "Abyss" had long sat among the lower planes. No one, including the gods, know how it came to be, but it is said to be infinitely deep and impossible to navigate. The Blood Chief came to call The Abyss home. Here he was free to create as many of his demonic monsters as he wanted. Amongst the infinite layers of The Abyss, The Blood Chief continued to raise his army anew, plotting, always plotting.