Three origins for Selenim PCs...

In the first and second editions of the French version, all Selenim were former Nephilim. The third edition introduced a second origin, the natural Selenim, who appeared after the Great Awakening at the start of the 20th century (see “Return of the Magicians”, Nephilim rulebook p77). I am incorporating this into my homebrew settings, but with some adjustments for the US version’s changes and my own ideas.

The Codex des Selenim divides the incarnated Selenim into three origins: contaminated, sacrificial, and natural. The contaminated and sacrificial are both former Nephilim, whereas the natural were always Selenim. Each origin has its own minor benefits and drawbacks compared to the other origins.

Natural Selenim are perfectly comfortable with their state, whereas former Nephilim always experience some lingering regret. Some desperately seek to find and restore their sundered Pentacle, locked in a Stasis they can no longer track. (Note: When a Nephilim loses a Pentacle branch due contamination or sacrifice, it is automatically sucked into their Stasis item and displaces any and all charges of that element.)

Ex-Nephilim

There are two ways for Nephilim to become Selenim: a gradual contamination by the Black Moon or sundering their Pentacle with a Shadow Blade.

Perpendicular to this are three categories, distinguished by the circumstances of the sundering:

• The first category, by far the largest, are those involuntarily infected and sundered one element at a time by unlucky natural exposure. It is not uncommon for these Nephilim to commit suicide before the transformation completes, via Orichalc or a suicidal assault against an overwhelming enemy. As sensitivity to contamination is determined by the level of Moon-Ka, most of them are former Onirim. They are sometimes known as “wild” Selenim.
• The second category, much smaller than the first, are those who were forcibly transformed for whatever reason. The vindictive revenge of another Selenim using Third Circle Conjuration or Necromancy, the Cult of Lilith's callous recruiters with Shadow Blades, etc. They never take well to their new nature, but most get used to it eventually. Unlike the attempted suicides of those contaminated, when the fait accompli arrives they find their survival instincts taking over.
• The third category is the smallest, bringing together those who voluntarily transformed: quickly and efficiently with a Shadow Blade, slowly and clumsily via deliberate infection one lunar sacrifice ritual at a time, or even assisted by a conjurer or necromancer of the Third Circle. Very few Nephilim are willing to sacrifice their connection to the magic fields, even among the most depraved Devils. This is almost always as a last resort method to escape being hunted down by secret society agents, "cure" extreme Khaiba infection, alleviate the agony of partial Elixirfication, or simply because the Nephilim grew tired of searching in vain for Agartha. 

…from a Contamination

These Selenim witnessed the slow sundering of their Pentacle under the Black Moon's contamination. They bitterly regret the transformation, expressing as melancholy and/or violent impulses. Curiously, their contamination developed its own minimal sentience…

Benefit: Pay 1 less Black Ch'awe to manifest the Imago's Aspects. Their Imago inherited their contamination's sentience, resulting in greater independence.

Drawback: −1 circumstance penalty to casting Conjuration Evocations. The Entities are close to the sentience that sundered their Pentacle, so summoning them always briefly reminds them of it.

Note: These modifiers are very situational. The Selenim won't get any benefit unless he specializes in Anamorphosis, and is likewise disincentivized to learn Conjuration. Necromancers ignore it entirely!

…from a Sacrifice

These Selenim used, or had used upon them, a Shadow Blade to quickly sunder their Pentacle. The sudden loss of their ability to manipulate the Elemental Fields renders them unstable and bipolar, yet oddly sensitive to their lost connection. 

Benefit: Immediately distinguish Elemental Ka in Ka-Vision without scrutiny checks. They retain a vestige of their mastery of the Elemental Fields.

Reduce Black Moon astrological penalties by 1 degree. Their forced transformation forged a weaker link to the cycles of the Black Moon.

Drawback: Reduce Black Moon astrological bonuses by 1 degree. Ditto.

Natural Selenim

These Selenim were born as proto-Selenim in the lower planes, who later merged with the astral self of a mortal dreamer who wandered in during a nightmare or near-death experience. They had to compete with numerous other proto-Selenim to escape the lower planes, which left them with a strong survival instinct. As they were born from the Tenebrae rather than transformed later in life, they have a more intuitive understanding of it.

Benefit: +2 circumstance bonus to Assuaging negative emotions. Their birth in the lands of nightmares gave them a profound understanding of humanity's worst fears and desires.

If using Tenebrae as a distinct technique, then natural Selenim start play with it at Master level for free. (See Vision-Ka #6 for details.)

Drawback: −1 circumstance penalty to Initiated tests within the lower planes. The natural Selenim are terrified of losing their Simulacrum and returning to the lower planes.

Further details...

Natural Selenim are former muggles who woke up one day incarnated as Selenim. They were never Nephilim and so don’t understand what the former Nephilim lost as a result of the Sundering. The natural Selenim were born from the residual Black Moon fields ever since the Fall of the Black Star, but remained rare compared to former Nephilim to the point they were considered legends. After the ascension and subsequent explosion of the Xibalbans’ Black Moon crescent in 1521, their population increased and the other Immortals really took notice of them for the first time. The Spiritualist movement in the 19th century and Crowley’s disruptions in 1900 further boosted this mass awakening. Some believe that the Great Awakening of all Immortals is caused by the coming of the Age of Aquarius.

Natural Selenim lack the biases of former Nephilim and have a more intuitive understanding of the Tenebrae. The Black Moon field doesn’t just parasitize the Moon field, but to lesser degree all the elemental fields. This understanding helped the Unnamed Arcanum develop many new spells, especially Tenebrae songs of Third Circle Conjuration!

The Ar-Kaïm of Serpentarius may be related in some way to the natural Selenim. Ar-Kaïm are "born", or first incarnate, under a Grand Enthronement of their Zodiac House. There is no Grand Enthronement of the Black Moon, so how Serpentaries first incarnate remains a mysteries. It is known that, like the Black Moon being a parasitic field that insidiously insinuates itself within the other magic fields, the Serpentarius is a parasitic House that hides itself within the other Houses. Do Serpentaries incarnate under conditions when the first incarnation of an Ar-Kaïm coincides with the Simulacrum incarnating as a proto-Selenim? These mysteries are disturbing...

Hope you enjoyed!