Creation stories can be very interesting, but the WIDE variety of gods and races in D&D I wanted to create some kind of organization or flow that allows for such a melting pot in the world. Here is how I made sense of everything for Miderra.
Miskah is the progenitor god. This creature of chaos did nothing but create. Most of these creations where horrific abominations that reside in the world of dreams. However, Miskah created a slew of gods during eternity. Whenever these gods came together they created universes of their own. Mimicking the raw creativity of Miskah but with their own spin.
Each of the universe was unique with its own cosmology and natural laws. Miderra, the home universe of my campaign had just four gods in the beginning. Essul the Verdant Wrath gave form to the world. She created its land, oceans, and beasts. Thudebar the Song of Storms breathed a soul into her creation and gave them desires and a soul. Asaj the Architect gave them reason and taught them how to achieve their goals. Finally Roth the Gavel saw the chaos and brought order and justice to the world through civilization.
These 4 gods lived alone with their creation for millennia. One day, a pantheon of gods from a different universe came to Miderra and begged for asylum for themselves and their creation. Their universe was consumed in a demonic invasion. They swore to help protect Miderra for sanctuary. The 4 gods accepted, and the foreign pantheon taught them how to protect their divinity to stop the demons from consuming it.
After that initial pantheon joined Miderra, several others followed.
Over time, the gods began bickered over the amount of influence they allowed each other, and used their peoples to spread their own influence and undermine their rivals in the material world. They soon forgot about the demonic threat.
Miskah loved the demons unifying power and sought to enter Miderra to end their squabbles and bring order among the gods, but Loki, a powerful giant, tricked the mortals into enacting Ragnarok which killed Miskah and all of its children. Effectively killing all of the old gods and scattering the demons throughout the world of dreams.
Without the gods maintaining the multiverse, they crashed together, shattered, and bled into each other. Walking from one world to another became substantially easier and forced thousands of species to interact for the first time. This is the present world of Miderra
The world of Miderra is home to hundreds of unique people groups. Given the fractured nature of the world it is not hard to find a settlement with its own unique combination of regional races. However, the various races can be sorted by their origin which is fairly common knowledge.
Dwarves, Gnomes, Halflings, Others
Many races that currently reside in Miderra were not originally from there. Many made their way here fleeing the death of their own world. Others were brought as slaves to work for tyrannical masters. Some may have simply gotten stuck here when the cosmology shattered. Regardless of how or why they are here, these displaced races do whatever they can to survive in this inhospitable world.
Every true dragon is a god whose lineage you can trace back to a single universe. The dragons grew in numbers and several dragon gods migrated from their home world to others for space. They once knew how to create life from the magic that flowed through their veins, however once their caverns were filled with these self replicating servants they no longer needed the magic, and the knowledge was forgotten. All draconic races know that the blood of draconic gods run through their veins.
Bugbears, Goblins, Goliaths, Half-Orcs, Hobgoblins, Orcs
The original races were crafted by Miderra’s original four gods. Bugbears, favored by Essul, reflected the animalistic cunning of the beasts she admired. Orcs, favored by Thundebar, were instilled with much emotion. They were prone to much mirth and rage. Goblins, favored by Asaj, were blessed with a curious and crafty spirit which allowed them to survive next to their larger kin. Giants, and goliaths, favored by Roth, instilled a desire for order in his people and the strength to enforce it. Over time, and with outside influences, the native races lost their overpowering influence in the world, but ruins of their once mighty kingdoms can still be found.
Human, Tiefling, Aasimar
These are the peoples to come to Miderra during the great Migration. When a celestial or fiend gives birth in Miderra it always results in the birth of a Aasimar or Tiefling respectively. If an aasimar and tiefling have a child, the result is always a human. An aasimar or tiefling only has a 50% of having a like-race offspring with a human, and an aasimar or tiefling that has a child with another of its kind will always result in an offspring of the same race. Their divine heritage has distilled in their blood making them as adaptable and diverse as the beings they descend from. However, even with the death of the gods and the collapse of the divine order many old world resentments still fall on these races. Humans, tieflings, and aasimar are also collectively called nephilim. They also culturally loath aberrations and demons since they are the source for all their historical woes.
Elves, Half-Elves, Genasi
Some locations in the world are so thick with magic it literally spews forth life. These races take on the physical qualities of the magic that birthed them. These traits in the past were more mellow, but after Ragnarok the deluge of magic has caused wild variations in physical traits of members of these races.