Eternals

Sometime after a child is conceived, an immortal spirit known as an Eternal will inhabit the child, effectively reincarnating itself into that body. This child will then be born with what is called the Mark of the Eternal. The mark acts as a signature for what spirit happens to be inhabiting the body and is almost always found on the back of their neck. A particular spirit will always leave the exact same mark on each body it reincarnates into. Because of this, the surname that is given to a Marked is the name of their particular Eternal.
A person who bares this mark will not retain any memories of their past lives. They do, however have quirks of some sort that carry over to each life they live. This could be something as simple as always having the same favorite color or always pursuing a particular profession. There are also more extreme habits that could present themselves like being prone to addiction or having a stammer.

Marks of the Eternals

While all Eternals are born with the signature birthmark that identifies their spirit, few Eternals are born with an awakened mark. Cosmetically the mark has more detail than a passive one but an awakened mark allows that individual to draw power directly from their spirit. One born with an awakened mark is considered nobility and is given privileged status among Eternal societies.
As these nobles grow and develop, so does their mark, furthering how much power they can harness from their spirit. By the time a noble has reached adulthood, their awakened mark will have most likely developed into a greater mark which acts as proof of nobility in Eternal society.
Awakened marks bare a second signature that surrounds the original. This second signature serves to identify which type of awakened mark they have. Specific marks will only awaken on specific races and provide similar boons to those who have them. Those who share the same awakened mark are considered spiritual siblings and those who have them make up the noble houses of the Eternals.

Lesser Marks

While those considered nobility are born with awakened marks (at the time considered lesser marks) which would later develop into greater marks. Eternals born with only a passive mark have a chance of developing a lesser mark sometime around or after reaching adulthood. This does not grant those who have these marks any title of nobility.
The noble house tied to that particular mark may seek that individual out and offer them a higher position among their guilds. Rival houses may also seek to recruit them as well to make use of their talents as well as use them as a bargaining chip or spy.

Aberrant Marks

Aberrant marks are a dangerous and more destructive manifestation of an awakened mark that can appear on any Eternal that has a passive mark. During a time of great grief, anger or pain, an Eternal can manifest an aberrant mark which will hold a corrupting influence over one who bares it.
Those publicly known to have manifested this mark are hunted down and executed since the mark is considered too dangerous to be left unchecked. Some will even attempt to manifest this mark on purpose in the name of revenge or simply because they seek power, no matter the cost.
Pure blooded orcs are the only Eternal-bound race that does not naturally develop awakened marks and as a result, do not have any claim to nobility. This has resulted in orcs effectively being classified as the runts of the Eternals despite them being the largest race in terms of size. Orcs are more prone to awakening the corrupting mark known as aberrant marks.

Eternal but Finite

There are a limited number of Eternal spirits that immigrated to the world of Draumur during the Great Clash. This number never increases with the means of how the Eternals were created being speculated at best. If a child is conceived while all of the Eternals exist within an already inhabited body, that child is born as what would be considered a normal person. A person born without an Eternal reincarnating into them is considered hollow and because of this are called Shells.
A Shell is considered a second class citizen and were commonly living a life of servitude during the first age after the immigration. The Shells themselves are thought to either not have an afterlife at all, this often being referred to as the Void, or their souls are condemned to an unknown fate. Many speculate that the original soul is ejected from the body when the Eternal inhabits it, leading to many Shells who give birth to a marked one not seeing that baby as their true child. These unborn souls are believed to be condemned to the Void, but many choose to believe that the souls of the unborn must go somewhere.
Eye colour is another constant when it comes to Eternals since a particular spirit is always born with eyes of the same color. Because of this, many believe the eyes to be a reflection of their soul. A shell is instead born with what are called colourless or pale eyes which can be various shades of grey. Yuan-ti are unique in the eye department in that they are normally yellow or orange, leading many, themselves included to believe they are no longer shells but a separate species entirely 

Taken Children

Many Shells would see their opportunity to become parents denied should they give birth to an Eternal. When a Shell becomes pregnant, they are to report this to a city official who will take measures to find out roughly when the baby will be expected. A member of House Orinthia will be present during the child's birth and that child will more often than not be taken if it is born with the mark of the Eternal. A faction of House Orinthia known simply as the Archivists will, at a later date, retrieve the child and take them to the Academy where they would be raised and educated.
Despite it being a crime, it is not uncommon for would-be parents to conceal the fact that they are expecting a child. Having knowledge of an individual hiding a pregnancy is punished just as harshly as the culprit themselves.
Some races such as the kalashtar, draconians and yuan-ti are so far changed from their original race that the Eternals are not capable of inhabiting them. The kalashtar are less widely known than the latter of the three while yuan-ti are constantly branded as monsters and hunted as a result. Draconians are quite a rarity to see but have been gradually becoming more common as generations pass. It is not uncommon for people to seek out a mate from one of these races in the hopes of saving their child's soul from the Eternals.
Other half-breeds such as genasi and half-elves are, for reasons unknown, still compatible with an Eternal spirit.

Eternal Afterlife

When an Eternal dies, it's soul travels to the Nexus Eternum, a magical device thought to be created by a forgotten god for the purpose of containing an Eternal's soul. The souls remain in a state of slumber until such a time that a compatible child is born for the Eternal to reincarnate into. The Nexus is contained somewhere within the walls of the mysterious facility known as The Academy.

Hidden School

Far off in a location unknown, infant Eternals are taken to a facility that is among a collection of small islands where they will spend their adolescent years. The children are taught by the by the educators of House Orinthia within the campus walls. An Eternal usually picks up on the skillset they have had over countless generations quite quickly and their education focuses on that particular skill set. One particular Eternal might have a knack for leadership in combat and as such, be taught in the art of war.
The methods of educating in some fields within the Academy are rumored to be quite dangerous. The academy supposedly keeps all kinds of monsters on site to give those being trained specifically for combat a true fighting experience. Shells commonly believe that any deaths of the students there are seen as little consequence in the eyes of the educators.

Graduation

An Eternal could leave the Academy anywhere from their early teens or mid twenties. When leaving, an Eternal has the option of choosing where they would like to begin their life in the world. After leaving the Academy, an Eternal is unable to return except in rare cases when the Archivists are willing to bring them back.