Ether

Ether is a white, gaseous substance that Fallen depend on to survive, and which only Servitors can currently produce. Ether Seeds may be processed Ether for later usage or storage, and Ether Drops are ether remnants.

In Destiny killing Fallen with headshots causes a torrent of ether to erupt from their heads. In Destiny 2 this takes the form of a momentary, ghostly silhouette of the dead Fallen.

Overview
Ether is apparently both a gas and a liquid, processed from energy and matter (organic compounds, in one known case) by Servitors through an unknown process. It is absorbed through breathing, and appears to run through Fallen bodies like blood. It can be even consumed by other beings as well. It is hypothesized to be an "ethereal lifeblood" for the Fallen but not a "soul" as is commonly assumed. When there is enough in quality it fills an Eliksni with strength. It is also cold. Ether was once freely available on the Eliksni homeworld but is now strictly rationed according to rank. Kells hold a device called The Elder Cipher, which controls how much ether is distributed from Servitors; Archons and Barons get an exceedingly large portion while Dregs get only enough to stay alive. This rationing stunts the potential strength and size of contemporary Fallen, particularly the lower ranks.

Certain Servitors are specialized in the distribution of ether among many Fallen crews, and glow when producing it They may be revered as gods to the Eliksni.

Ether is poisonous to Hive and possibly humans as well from what Failsafe said on Nessus. Although for the former, it is possible for hive to grow tolerant of Ether through breeding processes, as a Knight possessed Ether inside him.

Enhanced Ether
During the Red War, the Fallen House of Dusk was able to utilize stolen Cabal technology and the Servitor Serkuleks, the Fount to create a modified, more effective form of Ether known as "Enhanced Ether". Those who took this Ether substance would become Empyrean Fallen, exhibiting several effects including increased strength and health, and the side-effect of glowing like Servitors. Raksil, Archon Priest would head this operation before being killed. The project would later be put to a complete end following the destruction of Serkuleks at the Guardian's hands.

Dark Ether
The Scorn thrive off of Dark Ether, a disturbing, corrupted form of Ether derived from remnants of the Darkness afflicted within Uldren Sov from his experience during the Battle of Saturn. After wishing to save a dying Fikrul, which is fulfilled by the Ahamkara Riven, some of the Darkness within Uldren was transferred into Fikrul's lingering Ether, bringing the disgraced Fallen back from death and creating Dark Ether.

Dark Ether is capable of resurrecting dead Fallen at the cost of severely mutating them, such as how the Screeb units of the Scorn have been warped into quadrupedal Thrall-like monstrosities. It lingers instead of dissipating like normal Ether and is described by Variks as a fog-like gas that you move through like water, heavy and opaque.

Trivia

 * Ethers are a class of organic compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula R–O–R′, where R and R′ represent the alkyl or aryl groups.
 * Dimethyl ether is the organic compound with the formula CH₃OCH₃, simplified to C₂H₆O. The simplest ether, it is a colorless gas that is a useful precursor to other organic compounds and an aerosol propellant that is currently being demonstrated for use in a variety of fuel applications. It is an isomer of ethanol.
 * In literary, ether mean the clear sky; the upper regions of air beyond the clouds.
 * In Archaic physics, ether is a very rarefied and highly elastic substance formerly believed to permeate all space, including the interstices between the particles of matter, and to be the medium whose vibrations constituted light and other electromagnetic radiation.
 * In the lore of the “A Thousand Wings” Jumpship in Destiny 2, the Ether is called the Fallen's demiurge (Which equates to Survival).
 * It's possible that the Ether is also used to power some of their machines, notably Shanks, as they are able to be buffed by Servitors, as well as dropping Fallen Ether pick ups, and Ether Seeds, if it's a Major version. Servitors can also be buffed by nearby Servitors as well. But it's unknown why the Fallen use Ether to power their Machines, unless it was their only solution to power their machines, a tech flaw (since the Fallen using their only thing to keep them living, substance to power their machines, which would possibly waste their Ether supply, since it's very limited), or a strategic advantage.
 * This may be reinforced by the existence of the Hemolymph Repair Shank, designed to aid and fix Fallen Walkers.

List of appearances

 * Destiny
 * Destiny 2