The Missing Faction

A (political, cosmological or similar) system is made up of certain factions. But at least one of them is missing, its absence leaving a big hole that generates a lack of balance and harmony.

When such a faction is missing, don't be surprised if there is one member left.

A faction making a Face Heel Turn or Heel Face Turn qualifies only if its change of alignment leads to it no longer being a part of the system. If it's still a part of the system (although now in an antagonistic way), it's not this trope.

Comic Books

 * In Sandman, there are seven Endless: Destiny, Death, Dream, Desire, Despair and Delirium. The seventh Endless? Used to be Destruction, but he quit.

Film

 * Both Star Wars trilogies feature a universe where the state of galactic politics is based on the absence of an ancient order. In the original trilogy, it's the Jedi Order. In the prequel trilogy, it's the Sith Order.

Literature

 * The Bible has the Lost Ten Tribes of Israel.
 * In the Warrior Cats series, one of the five Clans, SkyClan, was forced to leave because of the humans' intrusions on their territory, and they eventually died out in their new home. The Clan was eventually rebuilt, but they are no longer part of the system; only a few cats even know of its existence.
 * In the Dresden Files, the sudden extinction of the late in the series' run has thrown the political balance of power in the supernatural world way, way out of whack.

Live Action TV

 * The Thirteenth Tribe from both versions of Battlestar Galactica.

Mythology

 * There are several tales of a Lost Roman Legion, which went to subdue a foe, never to be heard from again. Notably, this is one of the concepts Jim Butcher uses in the Codex Alera series.

Tabletop Games

 * The Old World of Darkness had several of these:
 * The lost/extinct vampire clans in Vampire: The Masquerade, namely, the Cappadocians and the Salubri. The power vacuum they left behind is filled in by the Giovanni and the Tremere in the Final Nights.
 * There's a clan in the Sabbat sourcebook called the Harbingers of Skulls who are pretty obvious the Cappadocians, though. To the point one of their Stereotype Reactions is from a recently-deceased Giovanni: "Oh, sh-"
 * From Werewolf: The Apocalypse, there are Apis [werebulls], Camazotz [werebats], and Grondr [wereboars], who were wiped out by the Garou [werewolves] during the War of Rage. Among the Hengeyokai (the shapeshifters of Asia), the Akuma--Asian werebears--are extinct. Also, among the Garou themselves, there are the extinct Bunyip and Croatan tribes, as well as the Black Spiral Dancers, who fell to the Wyrm.
 * Mage: The Ascension: For the Technocracy's predecessors, the Order of Reason, there were the Craftmasons (mages of the common people) and the Ksirafai (secret police). For the Council of Nine, there were the Solificati (alchemists) and the Ahl-i-Batin (Middle Eastern mystics). The Technocracy's Sons of Ether and Virtual Adepts don't qualify - they switched to the Council.
 * Wraith: The Oblivion had a few Guilds that were dismantled by the Hierarchy for the sheer fact that their Arcanoi were open to abuse and anarchy - the Alchemists (who practiced Flux, which allowed them to transform matter in the Skinlands), the Mnemoi (who practiced Mnemonsynis, which manipulated memories; they were originally judges before they did something very bad), and the Solicitors (who practiced Intimation, which involved manipulating wants and desires - which is powerful, as Passions and Fetters entirely define wraiths).

Video Games

 * Vega Strike: The Lightbearers.
 * Golden Sun: A lot is made about the Anemos tribe, of which two major characters are descendants and whose entire city apparently lifted off to become the Moon. Guess who doesn't show up in Dark Dawn?
 * The mythical 11th clan of S'pht, the S'pht'Kr from Marathon. On a related note, the S'pht homeworld's mythical third moon, K'lia.
 * The Androsynth in Star Control. Do not ask the Orz what they have done to them, and why.
 * The fifth house, Hispania, in Freelancer. What actually happened was their colony ship sputtered out before it reached a habitable planet (unlike the Liberty, Kusari, Bretonia, and Rhineland colony ships), so its survivors and descendants became space-faring pirate clans.
 * The backstory to Homeworld, which is literally All There in the Manual, goes into great detail about how Kushan society is broken up into tribe-like structures known as Kiiths. Every Kiith participated in the construction of, and has a place on the mothership intended to take their people back to Hiigara--except for Kiith Gaalsien, which was wiped out in a sectarian war. And don't even get started on the Kadesh.

Western Animation

 * In Avatar: The Last Airbender, there are four peoples, of which Water, Earth and Fire remain. The three of them used to live in harmony with one another and with the Air Nomads, but the Fire Nation attacked everyone and exterminated the Air Nomads.