Eldritch Being

"The Old Ones were, the Old Ones are, and the Old Ones shall be. Not in the spaces we know, but ‘between’ them, They walk serene and primal, undimensioned and to us unseen."

- H. P. Lovecraft, "The Dunwich Horror".

A term popularized by the Cthulhu Mythos, the word "eldritch" means something like "coming from a foreign realm". These beings are the strangest the universe has to offer; even for alien standards they're otherworldly. They are usually extremely powerful and typically display inexplicable and supernatural abilities.

They may manifest themselves either corporeally or noncorporeally. In some cases, their physical manifestations constitute a mere shadow of what they truly are, and they may have several avatars.

Often, beings such as these tend to be neutral towards lesser life forms, but they might be extremely malevolent and pure evil (e.g. Nyarlathotep, It, the Great Intelligence, Mind Flayer, Eihort and Y'golonac), or in rare cases they might be friendly (e.g. Mr. Nobody).

Types of Abominations

 * Adorable Abomination:
 * Angelic Abomination:


 * Animalistic Abomination:
 * Botanical Abomination:
 * Humanoid Abomination:
 * Mechanical Abomination:

Examples
"It was a terrible, indescribable thing vaster than any subway train—a shapeless congeries of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and un-forming as pustules of greenish light all over the tunnel-filling front that bore down upon us, crushing the frantic penguins and slithering over the glistening floor that it and its kind had swept so evilly free of all litter."

- William Dyer describing a Shoggoth.

Keep in mind that all examples are, at least to some extent, subjective, as there's no clear line between "eldritch" and simply "alien". The idea of what qualifies as an "eldritch abomination" ought to vary from person to person.

Literature

 * Cthulhu Mythos:
 * Great Old Ones (e.g. Cthulhu, Tsathoggua, etc.)
 * Outer Gods (e.g. Azathoth, Nyarlathotep, etc.)
 * Shoggoth
 * Colour Out of Space
 * Elder Gods
 * Flying Polyps
 * Hounds of Tindalos
 * Space-Eaters
 * The City and the Stars
 * The Mad Mind
 * Vanamonde
 * Humanx Commonwealth
 * The Great Evil
 * Vom
 * The Hog (Carnacki, the Ghost-Finder)
 * Pan (The Great God Pan)
 * Monstrous Sawfly (The Residence at Whitminster)
 * Anabis (The Voyage of the Space Beagle)
 * Thing (Who Goes There?)
 * IT/Pennywise (Stephen King's IT)

Comic books

 * DC:
 * Dark Giants
 * Star Conquerors
 * Marvel:
 * Dormammu
 * Shuma-Gorath
 * Many-Angled Ones
 * Ogdru Jahad (Hellboy)
 * Wolochs (Valérian and Laureline)

Film

 * Mr. Shadow (The Fifth Element)
 * King Ghidorah (Godzilla: The Planet Eater)
 * Calvin (Life)

Television

 * Doctor Who:
 * Animus
 * The Great Intelligence
 * Nestene Consciousness
 * Sutekh
 * Great Vampires
 * Eternals
 * Fenric
 * Reapers
 * The Beast
 * Weeping Angels
 * House
 * Insects (Lexx)
 * Mind Flayer (Stranger Things)
 * Supernatural:
 * Angels
 * Leviathans
 * Yokoth and Glythur
 * The Darkness
 * The Shadow
 * Ben 10: Ultimate Alien:
 * Lucubra
 * Dagon
 * DC Animated Universe:
 * Old Ones
 * Futurama:
 * Yivo
 * Brain Spawn
 * Four-dimensional Space Whale
 * Final Space:
 * Mooncake
 * Arachnitects
 * Titans
 * Invictus
 * Werthrent
 * Bill Cipher (Gravity Falls)
 * Sleeping Beast (He-Man and the Masters of the Universe)
 * Evil Entity (Scooby-Doo)
 * Dark Ones (Super Robot Monkey Team Hyperforce Go!)

Tabletop Games

 * Pathfinder:
 * Rovagug
 * Qlippoth
 * Dominion of the Black


 * Magic: The Gathering:
 * Marit Lage
 * Eldrazi


 * Rifts:
 * Splugorth
 * Vampire Intelligence

Video games

 * Sonic the Hedgehog:
 * Solaris
 * Time Eater
 * Giygas (Mother/EarthBound)
 * Flood (Halo)
 * Lavos (Chrono Trigger)
 * Brethren Moons (Dead Space)
 * Mass Effect:
 * Reapers
 * Leviathans
 * Thorian

Others

 * SCP Foundation:
 * Old Gods
 * Pattern Screamers