Pre-Adamites

"You see before Adam, before Devils flourished, and even before Angels. When there was only God, the Devil, and Death God told us about his plan to create more children he told us to play nice with them but we had different plans. We devoured them whole. Then God tossed us in the darkest pit he could find and threw away the key. But you see, you're the new key."

- A Pre-Adamite explaining it's history The Pre-Adamites also known as "Possessors", "Mistakes", and "The Oldest ones" are one of the multiple races in DxD. They are one of the oldest creatures in DxD being created directly by God even before God created the angels, from their history it is revealed that God created another race right after creating them but they devoured them forcing God to seal them away in a dimension called "Purgatory's Box".

Summary
After his existence with God and Satan, God created the Pre-Adamites. These beasts were among God's earliest creations, a race of creatures that came long before Adam and Even (hence the name Pre-Adam) but it is revealed that they came even before angels and devil races. God worried they would consume all the other species as they proved to be too strong, too hungry, and too destructive. Some time before he created Angels, God locked away them in Purgatory's Box using 77 Seals in order to prevent them from being released. When he had the Word of God tablets created, God made one for the Leviathans in case they ever escaped, putting instructions for how to seal them back up but this tablet was destroyed by the Original Satan during the First Great War.

The Pre-Adamites were created when there was only Gods, Satan, and Death. Due to being locked in Purgatory's Box they have no clue that God is dead and that Satan and God waged a war on each other because the two were originally friends according to them.

Due to being locked in Purgatory's Gate, they lost all forms of their original bodies, but due to their souls being immortal their souls still lived on while their bodies perish, this is why they are compared to ghosts in a sense but they are shown to be much stronger.

There are many ways of becoming a Pre-Adamite, from making a deal with a with one to having one possess you for so long that "you lose your humanity".

It is revealed that they are very hungry creatures, some wanting to devour anything they can, wanting to turn the human race into their own farm. Despite this, there are some who wish to co-exist with the human race and other mythological factions, these Pre-Adamites are labeled "sympathizers" and are killed by the normal Pre-Adamites. They are capable of emotions, but their emotions are like a switch, they can shut it off or keep it on. Those who want to live with their emotions share the name "sympathizer" with the ones who want to co-exist with other species and are seen as a threat to the other Pre-Adamites and thus are killed.

The exact number is hard to guess but some speculate that there could be millions of them. And the fact that they can turn humans into Pre-Adamites allows them to increase their numbers but they can't reproduce sexually or asexually instead having to rely on reincarnating humans.

Appearance
They manifest on Earth in the form of a long, spirits that resemble clouds with skull-like faces. Some are shown to still have their hair and even their eyes but most of their faces have decayed due to being locked in Purgatory's Box for so long. The Eight Pre-Adamite Kings appears as the only exceptions,with their souls being different colors which marks their position as a king: red, pink, silver, green, gold, yellow, purple, and blue.

When they possess a human body, the body will reconfigure and reshape in order to resemble their old selves before they lose their bodies, the body they possess becoming like their old ones. Because of this, it is shown that they all have horns. They have no wings but possess the ability to turn into smoke in order to fly. Once they possess a body they can't leave it unless forced out by a prophet, the only exception being the Eight Kings.

Factions
The only two rankings are normal Pre-Adamites who crave destruction, devouring, and killing all other lifeforms and the Sympathizers who want to live in peace with other supernatural species and humans.

Eight Kings
The Eight Kings also known as "Eight Symbols of Power" are the eight strongest Pre-Adamites in existence. They were the first eight created by God and they were the ones who created the other Pre-Adamites. It is revealed that only one of them is a Sympathizer the gold king who is revealed to be remorseful for what his race did and wanting to start all over. Each King has their own legion made up of Mid/High Class Pre-Adamites and Low Class Pre-Adamites. They possess telekinesis, the ability to manipulate weather, the ability to cause curses, and a wide array of powerful abilities.

Middle/High Class Pre-Adamites
The Middle/High Class Pre-Adamites are the ones strongest below the Eight Kings. They are shown to be frighteningly powerful. Some are shown to just be knights while others can be Generals and Commanders, there are many different sub-categories in this type. Their spirits are shown to be either white or black.

Jack Knights
The Jack Knights are very powerful Pre-Adamites chosen by the Eight Kings themselves because of their strength. They possess the ability to materialize weapons made of their energy out of thin air along with other powers.

Dealers
Dealers are shown to be powerful Pre-Adamites but they are more like businessmen who spend their time making deals with humans and other creatures in order to increase their numbers. They possess the ability to grant wishes (can cure cancer, heal injuries, and increase someone's strength) they can grant any wishes below bringing someone to life and making someone immortal.

Achilles
Achilles are powerful Pre-Adamites with the ability to transform into shadows, they can also manipulate shadows.

Alchemists
Alchemists are powerful Pre-Adamites with the power of alchemy (to fuse things together) and can even materialize a body without using a human as a vessel.

Hybrids
When a Pre-Adamite has sex with a human or a being of another species it creates a hybrid of some sort. The hybrids are shown to only take a full human day to reach adulthood. Despite this, just like Pre-Adamites they are immortal and stop aging after they reach early 20s. Hybrids are shown to possess telekinesis powers surpassing all other types making them the most powerful below the Eight Kings.

Low Class Pre-Adamites
The weakest and most vulnerable Pre-Adamites. Due to being the weakest they possess the least power and are typically foot-soldiers and slaves to the stronger Pre-Adamites. Their souls and forms are shown to be gray and the most Pre-Adamites come in this category. When a human chooses to become a Pre-Adamite, they are typically in this category.

Powers & Abilities
Powers varies between Ranking and Class

Holy Energy Manipulation: Being made from God similar to angels they possess the ability to manipulate holy energy. Low Class Pre-Adamites are shown to only be able to make claws and fangs out of holy energy while Mid/High Class can make wings out of holy energy.

Biologically Immortal: Being biologically immortal beings due to their souls being immortal they cannot die unless killed meaning illnesses have no effect on them.

Superhuman Strength: Even the lowest of Pre-Adamites can leap unnatural distances and lift objects far beyond the limits of the body they are possessing. Higher Pre-Adamites are overwhelmingly stronger than lower Pre-Adamites.

Consumption: Their hunger is one of their strongest trait, able to consume a whole race in a few days, shown when they consumed the other Pre-Adamite race made by God long before the creation of Angels.

Superhuman Speed: They are shown to be very fast and have heightened reflexes, their speed varies depending on their ranking.

True Eyesight: A ability belonging only to the Eight Kings, this allows them to see through the eyes of others under their ranking.

Kinetic Energy: High/Mid-Class Pre-Adamites are shown to be able to manipulate moving objects, able to make throwing a rock as strong as a speeding bullet.

Possession: Pre-Adamites possess the ability to possess a human body but once they are in the body they can't leave unless exorcised by a Priest. The only exception being the Eight Kings.

Trivia

 * The Pre-Adamites history is based on Leviathans in Supernatural excepted I added the terminology about them destroying another species.
 * Asterians are said to hate Pre-Adamites.
 * The Image above is Jellal possessed by Zeref from the Fairy Tail franchise.