Brainstorming

Shin and Wayin are dual creative deities. Together, they created all that exists. They placed on Earth living creatures with a limited lifespan. They also placed non-animate plants with limited lifespans.
Biologically immortal creatures:

• Tortoises (age incredibly slowly)
• Bristle-cone pines (5k+ years old; harsh environment)
• Quaking Aspen (clones itself; all part of the same tree root system, mutations over time)
• Clams (maybe)
• Lobsters (usually killed by exhaustion while shedding exoskeletons; DNA doesn't degrade)
• Hydra (microscopic clones; FoxO protein preserves their lives)
• Immortal Jelly (can revert to polyp form from maturity and regrow indefinitely)
• Bdelloids (zooplankton immune to extremes in temperature; hibernate and reawaken at will)
• Glass sponges (can control the speed of their micro-cellular growth, thus their aging)
• Lungfishes (can survive without water/nutrients for long periods of time; hibernation states)
• Water bears (survive extremes of temperature and radiation; hibernate until in contact with water)

Angels are created. They are biological creatures invented and designed by Shin. She designed them to be immortal and the ultimate chameleons, able to hide their wings and change their shape to camouflage themselves. They can be killed, usually by disease or physical and emotional trauma, plus they can choose to age and die as humans do. They reproduce through sexual oviparity, laying fertilized eggs after a period of internal incubation. Their genders are socially constructed, as they are genderless at birth and choose a primary presentation at maturity. They retain the ability to change gender at will with the rest of their shape-shifting abilities.
When Shin and Wayin fought and Shin took her creations with her, she left one angel behind, Serafiel. Wayin captured him and effectively dissected him in studying his design. From what he learned, Wayin created his own version of angels. Because his understanding was limited to a male-presenting individual, the resulting clones of Serafiel always present as male. Mutations are present in the cloning process, resulting in differentiation in identity, personality, sexuality and personal preferences. Like hydras, these clones are biologically immortal as long as they have access to a specific protein compound. Without this compound, they are subject to physical aging and rapid DNA deterioration, a condition known among angels as "Wasting Fall." Wayin's angels are supplied with the protein compound via their water supply.
Just prior to the Great Flood, two hundred of Wayin's cloned angels defected and left Heaven. They were led by Lucifer and aimed to take human wives and live among humans. They found humans amenable to this and settled into human culture for approximately a generation of human lifespans, after which their children, the nefilim began to challenge humans for control of the tribes of Earth. Outraged, Wayin sent the flood to destroy the nefilim and any humans who sympathized with them. He also sent out his assassins to kill any remaining angels. Lucifer caught wind of this plan from a sympathetic angel, Musiel, who had fallen in love with Lucifer's son, Nathaniel. Musiel joined Lucifer in an attempt to shield as many nefilim and renegade angels as possible from Wayin's wrath. They escaped into deep underground caverns which Lucifer and several of the other full-blooded angels with higher abilities shielded from Wayin's sight. They have lived in hiding ever since.

Additional science notes:

• Spotted salamanders use algae they eat in order to use photosynthesis to power their cells. This could be the source of photosynthesis in angels and would require that they consume calories on a regular basis in addition to sunning. It would make food intake a requirement rather than a choice, though they could gain less nourishment from general food rather than algal-based foods. It could also make them dependent on Heavenly sources for food, if they don't know the origin of their algal components and have them provided.

Second Thoughts

The story I want to tell is about a human society that includes two classes of angel: fallen angels who are distrusted and considered dirty or bad and "legit" angels who are considered pure and good. Neither perception is accurate, as angels on both sides have the same failings and strengths that humans do. Angels who are part of the hierarchy, "regimental" angels are expected to follow orders from their superior officers without question and to put the good of the hierarchy ahead of their personal well-being, "for the good of the flock." Regimental angels form military organizations which are legitimized by human leaders and governments.
Fallen angels are outcasts from human and angelic society. Humanity views them with suspicion, thinking of them as diseased, fundamentally immoral, or out-and-out evil. Regimental angels view them with anything from grief to disgust and aggression. There is a disease, wasting fall, to which only fallen angels are vulnerable and this reinforces the human preconception. Neither angels nor humans know the source of wasting fall, but it is suspected to be tied to reproductive behavior, probably sex with humans. Sex with humans is also largely considered the cause for angels falling, a sign of their inherent immorality and forbidden among the regimental angels.
This represents the status quo.
The status quo is disrupted when a third race appears: nephilim, the offspring of humans and angels. Any human could possibly be a nephilim and no one knows who might be one until they either "manifest" or are medically tested. Manifested nephilim go through a heat or rut cycle similar to an animal breeding season. This state drives them to be nearly mindlessly sexual, primarily toward angels or other nephilim. Manifestation can be triggered by any stressors: the hormonal changes of puberty, the excitement of a first kiss, the strain of being in a doctor's office, prep for surgery or a major exam. In order to locate nephilim before they manifest, angelic testing sites are sponsored by most world governments and humans are required to register there and be tested or risk arrest and punishment.
Nephilim are viewed by humans with fear and suspicion, dirty by association with fallen angels. Regimental angels barely consider them to be human at all and encourage humans to look the other way when their civil rights are concerned. Nephilim are located by angelic forces, detained and then usually vanish from human society. The angels claim they are being relocated to nephilim-only camps and are being treated well. In reality, most of them are killed and disposed of while many of the women are relocated to hidden testing sites run by some of the archangels.
Regimental angels in the rank and file fear falling. Falling means being split away from the flock and isolated, something to which most angels are not well-suited. Because the mechanics of falling aren't well understood, regimental angels believe that heavy interaction with nephilim puts them at risk of falling. In particular, while in heat, nephilim put out a pheromone which makes them incredibly sexually attractive to angels. Angels in the upper echelons encourage the rumors that say the pheromone triggers the fall, which allows angels to interact with humanity in a limited capacity with the use of masks.

Key Players

• a pair of fledge-mated regimental angels, one who falls in love with a nephilim and the other who resists leaving the flock.
• a newly manifested nephilim trying to escape the angels.
• leader of an angelic/nephilim underground railroad.
• human/nephilim scientist studying wasting fall.
• an older, sergeant-level regimental angel who begins to question.
• corrupt leader of the angelic forces (or regional unit)
• a female-identifying angel who is trying to locate her children/mate