The Creation Myth

"Before there was time, before there was anything, really, there was nothing. And before there was nothing, there were monsters." -Unknown

In the beginning, there was nothing. Emptiness. From that void came Eru, Everything. Law. Order. Stability. Stasis. Inan followed closely after. Inan, who was Chaos. Entropy. Decay. The young brothers pushed and pulled, and their play led to the creation of Aste, the mother of Creation. Within that primeval void, Eru and Inan both fell in love with Aste. Aste eventually chose Eru, which angered Inan greatly. Enraged, Inan attacked Eru. But before his sword met Eru, who had been caught unaware, Aste threw herself in from of Inan's blade. As Aste lay there dying, her life force gave birth to the primordial creation.
Eru and Inan then engaged in a vicious battle that lasted untold millennia over Aste's spilled blood. The battle shook and reverberated throughout the chaos. as they battled, their spilled blood mixed. Eventually, Eru was able to fatally wound Inan, but not before being run through by Inan's sword. The mixed blood of Eru and Inan created the Primordials. Eru's blood gave birth to Time, and Inan's blood created the Void, which would later be known as the Far Realm. As Time awoke to the sight of violence and mayhem, the dying Eru pleaded with Time to safeguard his love's creation, and Eru hurled himself and Inan into the Beyond to die. And Time was left, to govern the primordial chaos. 
Time then christened himself Ao, the Lord of Time. A ponderous being, Ao thought for eternities how best to bring order to the Chaos. The Primordials roamed in the Chaos, blissful and happy. Ao eventually devised a solution and created magic to shape Chaos. He saw his work and was pleased. However, Time had not foreseen how exhausting this act would be and fell into a deep slumber. 
While Time was asleep and the world lay still, two beings emerged from the Primordial Chaos. Jaziran and Ahriman. These beings, among countless others that sliced and died in the chaos, battled. Such conflict led to them becoming champions of law within the Primordial Chaos. Also born from this soup was Velhak, who was also known as Thrazidun. Velhak was a being that sought to depose Ao while he slept, and wished to rule in his stead. Jaziran and Ahriman knew this and chose to do battle with Velhak. Their cataclysmic battle led to what would come to be known as The Sundering.
Over countless millennia, different parts of the Primeval Chaoshad began to coalesce and group together due to the blood of Eru and Inan. As Jaziran, Ahriman, and Velhak battled, their clashes shook creation to its core, and had fractured creation. When creation was sundered, the Astral sea was left as the wound and reminder of the division of creation. The Primordial Chaos was split into separate layers, which would become known as the outer and inner planes. 
When Velhak sundered the world, Ao woke up. Furious, Ao slew Velhak. Velhak's blood pooled into the universe. Afterward, Ao was still weakened and unable to affect the Chaos to his liking, and so he charged the Elder Ones, the Primordials, to begin shaping the cosmos, creating the Vvelenfaar, the Proto-prime material plane. The Vvelenfaar was the prime material plane conjoined with the Feywild and the Shadowfell. 
From the Outer Chaos, the Gods began to form from Outer Chaos. These Gods began to shape the Outer Chaos to their liking, creating the outer planes. 
The Primordials were angered that Creation had been sullied with the blood of Velhak, seek to remake creation. The Gods, which had taken a liking to Creation and wished to protect and govern it, go to war with the Primordials. This conflict would be called the Dawn War.
As the Primordials did battle with the Gods, the life force of the slain primordials gave birth to the Obegar, the proto-mortals. As the Dawn War raged on and rattled creation, a race of beings that had grown within the Void known as the Obyriths fashioned a weapon, a shard called the Erudite. The Primordials would eventually lose this battle.
This shard pierced through the Void and allowed the Obyriths to seed the universe with the first source of pure evil within the Universe. One of the Elder Gods, a being known as Ordo, became corrupted by the touch of the Void and was driven mad. He became the Mad God, Sheogorath. 
The Obyriths then went to war with the Gods, in what would become known as the Abyssal War. The Erudite was wielded by the chieftain of the Obyriths, a monstrous being known as Hezorak. This war awoke Ao once again, who joined his children in battle with these monstrous beings. The Lord of Time and the Gods vanquished the Obyriths, and the Erudite was destroyed. In doing so, the Vvelenfaar was shattered, in what is known as the Second Sundering. The Vvelenfaar was shattered and split into the Prime Material plane and its countless planets, the Feywild, and the Shadowfell were all forcibly split into separate planes of existence. The spearhead of the Erudite plummeted through the bottom of the universe, eventually coming to rest upon a rock. This would create what would be known as The Abyss.
The Obyriths retreated to the abyss to lick their wounds. From the Abyss they learned that they could corrupt the Obegar's mortal souls and turn them. This knowledge created the first Demon, Demogorgon.
While the other Gods were concerned with the affairs of the mortals and ruling over them, Ahriman felt that the Gods should follow the Demons into the Abyss and exterminate them.
Ahriman took an army of celestials to Ao, and petitioned him to build Baator, a war station so that he might wage war on the quickly propagating Demons. Ao granted this request, with the caveat that the Gods will forget his identity as Ahriman. Jaziran did not wish to forget his brother-in-arms, and argued with Ahriman over this fate. Ahriman, ever noble, told Jaziran that he must let him destroy this evil. The two came to blows, in a battle that Ao deemed must be decided between the two. This battle between the eldest of Gods was one that reverberated throughout the Cosmos, with each snake trading blows that could shatter planets. Eventually, Jaziran became wrathful, and his attack sent the wingless Ahriman hurtling through the universe.
Ao ensured that the Gods would forget Ahriman. And so Ahriman, bitter and angry that he was betrayed by Jaziran, fell for eons. His plummet ended when he landed, dashed upon a floating spire. This spire would become known as Baator, the home of Ahriman. Ahriman, weakened and angry, began to plot. He called his army to him, appearing under a new moniker. And so, the Leader of the Nine Hells was born: Asmodeus.
The Gods then ensured that the Primordials were imprisoned within Carceri, a massive prison plane within the outer realms. They then proceeded to guide the mortals, hoping that the mortals would become better than their forebears. And so, the world as we know it was created.