A recap of the Selenim rules introduced in 3e

While the Selenim were given rules in the first and second editions of the French version, these were wishy-washy and the original creative leads preferred them as NPC-only (which once again became the case in 4e and 5e, much to my chagrin). In third edition Nephilim: Revelation, the publisher opted to revise their rules for playability and compatibility with mixed groups of Nephilim and Ar-Kaïm. Additional rules were provided in their respective Codex.

I recap the overarching structure of the 3e revisions here. I have divided this post into Initiation, Aspect, Sapience, Past, Downfalls, and Other details, mimicking the structure of character creation.

Initiation

Unlike the Nephilim's Pentacle of the five Elemental Ka, Selenim have only a Core of Black Moon-Ka. Thus, they can only perform their own Occult Sciences (see below).

Additionally, they have a Black Moon Pool that is spend to cast spells. They suffer a degeneration known as Entropy: the Pool automatically loses 1 point at solar midnight every night, doubled on the nights of the full and new moons. If their Pool ever falls to 0, then they lose 1 point from their Core every hour thereafter. If their Core falls to 0, then they die.

Selenim recover points by "Assuaging" (Assouvissement in French): siphoning Solar-Ka points from mortals, with the amount siphoned depending on the intensity of the target's emotional state. As such, Selenim are encouraged to find or cultivate strong emotional states in mortals to provide the sustenance they require.

If a Selenim fumbles an Assuaging or overfills during it, then they transfer Black Moon points to the target. These points decay at midnight as usual, but have deleterious effects in the mental state of the contaminated mortal: they become what is called a mort-vivant in French ("living dead" or "undead"). Necromancers may do this deliberately, as they may perform further spells on the living dead to make them useful familiars.

Aspect

Selenim lack a Metamorphosis like Nephilim have, though they can create an Imago that serves a similar purpose (see below). 

Selenim generally have one of two "Origins": Ex-Nephilim or Natural Selenim. Although only mentioned in the Players' Book, more concrete rules were provided in their Codex.

An Ex-Nephilim is a Nephilim who was transformed into a Selenim, by slow lunar entropy or by sudden sundering, voluntarily or involuntarily. The majority were transformed involuntarily by lunar entropy, the larger minority involuntarily by the actions of another, and the smallest minority voluntarily. They benefit from their past lives as Nephilim.

The Natural Selenim were never Nephilim: they were mortals who encountered a proto-Selenim in the second strata of the Anti-Lands (lower astral planes) while having a nightmare, and it came back with them to the waking world. They form a minority of Selenim. Thus far there have been two waves of such "Apparition" that PCs may be part of: in the late 19th century due to the Golden Dawn's disturbances and during the World Wars.

In The Selenim Codex, additional minor occult paths and unique character options are offered. These include the Ethereals, the Loa Guédé, the Orichalcians, the Unnamed Arcanum, the Cult of Lilith, and the 666.

Sapience

While the Selenim cannot practice elemental magic, they have developed three occult sciences of their own: Necromancy, Conjuration and Anamorphosis. The Players' Book details the first two circles, while the Gamemaster's Book details the third circle. A supplement dedicated to the occult sciences would have provided further details, including a Quest associated with mastering each science, but the publisher went under before it could be released.

Necromancy involves the transgression of the boundaries between life and death. The first circle allows the Necromancer to commune with the dead, extend their capabilities, and reanimate them as zombies or Sans-repose ("Restless"). The second circle allows the Necromancer to invoke a ghost to possess the living, project their own consciousness into the Astral, and contaminate the living with the Black Moon to create mort-vivant ("Living Dead"). 

The third circle, Thanatology, allows the Necromancer to affect Nephilim and Ar-Kaïm using Rare Earths (Litharge in the US version) as a connection: the Necromancer can read the target's mind and control them, even resurrect the target from total destruction. A Necromancer who masters the Thanatology Quest supposedly gains nine lives.

Conjuration involves the calling and creation of Entities from the Loess of the Pavane (the heart of the Black Moon fields in the deepest strata of the Anti-Lands). Depending on their specialty, these entities may affect the emotions of their targets, spy on targets and remotely relay intel, inflict direct physical damage, corrupt and erode Ka, etc. 

The third circle, Composition, allows the Conjurer to directly manipulate the Black Moon fields in a manner not unlike the Nephilim's spontaneous Sorcery. This affects not just Entities but mortals, Immortals and Restless. A Conjurer who masters the Composition Quest supposedly becomes able to perform elemental magic again.

Anamorphosis involves concentrating Black Moon points and constructing them into a sentient elemental creature (akin to a tulpa) known as the Imago, based on the Selenim's own personality and channeled through mundane portraiture. The Anamorph literally draws or paints their Imago in a studio, causing it to form from their Black Moon Pool! The Imago allows the Anamorph to manifest its aspects on their own body, granting them superpowers that make them a monster in combat and protects their fragile immortality. These aspects may have non-violent applications too, such as intellect or beauty. 

The third circle, The Realm, allows the Anamorph to create an entire pocket world around their Imago that reflects their own psychology. This "Cursed Realm" is anchored to and shadows a physical place on Earth, such as a building or desert, as its territory. The Anamorph can learn to affect the inhabitants of their territory, such as to influence their minds or even change their Simulacrum! The Realm is adjacent to the Anti-Lands, allowing the Anamorph to accept migrating Entities and influence them into service. The drawbacks of the Realm are that it is permanently fixed to its single territory and thus limits the Anamorph's movements, it must be sustained by Solar-Ka drawn from the inhabitants, and it is drawn to merge with one of the five branches spiraling from the Unknown Place. An Anamorph who masters The Realm Quest supposedly merges with his territory and becomes omniscient within it, perhaps retains sovereignty of his Realm when it drifts into the Unknown Place.

Black Moon magic is subject to astrological modifiers. Since this Ka has no Planet of its own, this is due to ricochet off the other magic fields that the parasitic Black Moon field insidious insinuates itself within. Thus, the modifiers are about halved compared to those applied to Nephilim. Black Moon exhibits astrological relations inversely to Moon, like an opposed element: months of Air and Fire are favorable, months of Earth and Water are neutral, months of Moon and Orichalc are opposed, and the month of Solar-Ka (Leo) is randomized each day. The current lunar phase further modifies this: the Full and New Moon is unfavorable, whereas the Waning and Waxing Moon are favorable. Thus the best day to perform black magic is during the Waning or Waxing Moon under an Air or Fire sign (or a favorable day under Leo), whereas the worst day is during the Full or New Moon under a Moon or Orichalc sign (or a opposed day under Leo). 

The Black Moon has no Enthronements or Grand Enthronements due to lacking a Planet, and this is a plot point in the text: since Ar-Kaïm are only (re)born during Grand Enthronements, this raises the question of where Serpentaries come from if their Dominant Ka is Black Moon. Oddly, some passages suggest the Full and New Moon are the closest equivalent of Black Moon Enthronements (cf. Realm's Anchor in GM's Book, Prime Essence in Codex Nephilim), in direct contrast to the unfavorable astrological modifiers. I can't tell if this was deliberate or an oversight; since this mechanic and the concept of Selenim PCs was dropped in subsequent editions, it's impossible to know the writers' intent now.

Past

Unlike the Nephilim, Selenim don't reincarnate. Instead, their Simulacrum ceases to age upon their Transformation or Apparition. Thus, they walk through the centuries rather than being obligated to enter long periods in stasis. Lest their ageless faces arouse suspicion, Selenim are forced to live in hiding or move from place to place and reinvent themselves as a new identity with every century that passes.

That said, the Selenim suffer a unique existential angst as a result of their physical immortality. Their Codex refers to this as the Limbos, Hells, and Abyss. Basically, they go through cyclical stages of grief, angst, and general mental illness. The only treatment for this is for them to mimic stasis by retreating from the world and into a Cursed Realm, an Anti-Land, or an Obsidian Stasis item for the Yohual-Tecuhtlin.

Their Codex includes options for purchasing Advantages at character creation. (Advantages were introduced in second edition, but were not included in the third edition Players' Book due to space constraints.) Beyond Skills and Occult Lore, these included Allies, Artifacts, Anti-Lands, Figures, and other assets retained from past lives that the PC could start play with.

Downfalls

Selenim are not allergic to Orichalc like Nephilim. While an Orichalc weapon can harm their Simulacrum and potentially kill them that way, it doesn't inflict magical damage to their Core. By using combative Aspects, an Anamorph can potentially slaughter a team of Templars armed with Orichalc swords and bullets at little risk to himself.

Selenim don't suffer from Khaiba, Narcosis or Shadow (Shouit). Selenim only really suffer from two unique degenerations: Entropy and Limbos/Hells/Abyss. These aren't rated like Khaiba or Narcosis is for Nephilim, and the latter doesn't receive concrete rules anyway. 

However, like all characters Selenim take Flaws at character creation to balance their choices during the preceding steps. They share many Flaws in common with Nephilim and/or Ar-Kaïm, but have only a handful unique to them: basically higher susceptibility to Entropy and reduced Black Moon Pool capacity. Ex-Nephilim may take some Flaws otherwise reserved for Nephilim, but Natural Selenim lack any unique Flaws.

Selenim normally lack a Stasis item and, due to its lacking functionality for them, may only take it as a Flaw (see below). If their Simulacrum is killed, then their Core perishes with it. As such, Selenim are typically a craven lot and rely on mortal and magical familiars.

Other details

Stasis

Ex-Nephilim may or may not still have their original Stasis item. However, it ceases to work for them. They cannot rejoin it upon disembodiment, tap it for charges, or track its direction. However, it contains their former Ka-elements: when their Ka-elements are sundered by lunar entropy or a shadow blade, the branches are automatically sucked into the Stasis item and remain there indefinitely.

Natural Selenim never had Stasis items.

Some secret societies have developed the means to bind Selenim to the Stasis items of destroyed Nephilim, but these don't function correctly for Selenim and thus are taken as a Flaw. The Selenim may track it as usual, but cannot tap it. If the Stasis item is broken, then the Selenim perishes immediately (this is true for all Immortals in the French version). If the Selenim is killed, then their Core is automatically sucked inside rather than perishing. However, as the time the rule was written there is no known way to release the Core to incarnate in a new Simulacrum, although the Unnamed Arcanum and Temperance were working together on a ritual of deliverance. Otherwise this entrapment is inevitably fatal for the Selenim, as the Core continues to suffer Entropy within the Stasis item until dissolved.

Although only alluded to in their Codex, the 2e supplement Exils explains that the Yohual-Tecuhtlin invented Obsidian Stasis items that would preserve their Core against Entropy indefinitely. Like the Nephilim's Stasis items, they are released by favorable Enthronements. However, concrete rules were never provided. Indeed, the Black Moon doesn't have Enthronements and its astrological modifiers are weaker than those of the other Ka (see above).

The 666 have developed rituals to bond a Selenim to a Stasis item with a Nephilim trapped inside, or to their former Stasis item with their branches inside, creating hybrid Immortals.

Simulacrum

Selenim don't relate to their Simulacrum the same way that Nephilim do. Something about the Transformation/Apparition removes the Simulacrum's will while preserving their Solar-Ka. Thus, Selenim never risk Shadow (Shouit) upon fumbling a mundane skill roll. 

Normally, the Selenim perishes along with their Simulacrum when he is killed. Selenim cannot reincarnate as easily as Nephilim due to their inability to survive disembodiment or release from Stasis, but that doesn't mean it is impossible for them. The Loa have a ritual allowing their cult to summon their Core from their current Simulacrum and incarnate it within a willing cultist, normally only used to save their bacon from emergencies. The Realm has an Aspect to transfer the Core from the current Simulacrum into a new one, which leaves the former Simulacrum in a zombie-like state. The Yohual-Tecuhtlin can survive the death of their Simulacrum by rejoining their Obsidian Stasis, and this preserves them against Entropy, but no concrete rules are given for how they reincarnate.

In addition to becoming ageless, the Selenim's Simulacrum has a vastly reduced metabolism. Their brain activity resembles someone in a deep coma, they don't need to eat or drink as often, they're somewhat less resistant to disease due to not producing antibodies, and their fertility is greatly reduced but not sterilized.

A Selenim can have normal biological children with a mortal partner. However, the child of two Selenim will be born contaminated with Black Moon-Ka. While these points will decay at the usual rate, the child will thereafter be more likely to encounter a proto-Selenim while dreaming and experience Apparition into a Selenim.

Hope you enjoyed!