Eldritch Abomination / Marvel Universe

Now this is an Avengers-level threat!

You're stomach bacteria! That's all you are to Gah Lak Tus! Gah Lak Tus is purity! It mantains the cleanliness of existence! It is the Antigod -It decreates intelligent life in the universe! Don't you understand? You're already dead! Gah Lak Tus returns all intelligent worlds to their original state! Only the Kree will survive because we have learned from each Gah Lak Tus event! You have to die so we can live!

— Ultimate Secret #4

Eldritch Abominations in the Marvel Universe.

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• Doctor Strange tends to fight these: Dormammu, the ruler of the Dark Dimension, and the most-powerful of the Faltine — a species of mystical entites manifested from magic; Nebulos, who can channel infinite energy and required the Living Tribunal to stop it, Satannish, a Reality Warper Hell-Lord of two faces and four horns; Nightmare, an Energy Being Made of Evil of the psychic plane that feeds on fear; Blackheart, Mephisto's son made from the pain of murder victims...
  • Special mention goes to Zom, an ancient living weapon with unspeakable power sealed in a magical amphora, that Strange has to let out to have any hope of defeating Umar. It works, scaring Umar (a Humanoid Abomination with near-godlike magical power nearly on par with those of her brother, the aforementionned Dormammu) shitless, and it takes the Goddamn Living Tribunal itself to put him back. Later on, during World War Hulk, Strange imbues himself with some of Zom's power and proceeds to beat the shit out of the Hulk before getting distracted.
• The Old Ones/Many-Angled Ones — namely Shuma-Gorath — are directly based on the Great Old Ones and Outer Gods of the Cthulhu Mythos; and are ancient multiversal threats. The most powerful known regular demon lords are supposed to be as insects to them, as they are stated to be only a step or two below the Living Tribunal. Carnage Vol. 2 counts the Elder God Chthon as the mightiest of them, portraying him as an expy of Cthulhu who was sealed away in the extradimensional realm of K'lay, and with the chant to summon him paraphrasing the iconic chant from The Call of Cthulhu.
• Galactus. Older than the universe itself and incomprehensible by mortals. It's Canon that his hat-wearing humanoid form is simply an image our puny minds superimposes over a reality we cannot truly comprehend; for instance, Beta-Ray Bill's species sees him as a big starfish.
  • His purpose is to keep Abraxas from killing everything everywhere in every parallel universe. The reason he goes back and forth from being depicted as able to go a long time without eating to only having enough time to reach the next planet is because maintaining Abraxas' seal is harder at some times than others.
  • Subverted in Marvel Zombies, where the zombified superheroes manage to physically devour Galactus' body. The easiest explanation is probably that, in such a universe, Galactus was just some sort of very powerful alien, filled with cosmic powers yet still made of an organic, physical body. Although this could simply be how you and the Marvel Zombies interpret Galactus getting eaten. That, or in this universe, Abraxas has already been dealt with (something probably ate him; that's just how it goes around here.)
  • Although he's a mechanical example, Ultimate Gah Lak Tus definitely qualifies. Aside from driving everyone who sees it insane and spreading a flesh eating virus across the planets it consumes, much of the devastation it causes stems from its own gravitational pull.
• Mephisto qualifies as this. Sure that he may look like a a red-skinned vampire, but that's just his most common form. In reality, Mephisto is an inconceivable embodiment of evil that rules a part of Hell's plane of existence, and he has gave a taste of what he truly looks here◊! It's implied that he might not have any form at all. Downplayed in that while he likes to act like he's a big deal, he's a lightweight compared to Set, Chthon, Dormammu, and the other really nasty beings out there.
• Hexus, the Living Corporation, from Marvel Boy, is a being native to another universe who is the Sentient Cosmic Force of Capitalism Is Bad, a sentient idea/psychic parasite that takes the shape of Brand Hex, a MegaCorp that brainwashes new employees to be its avatars, who then work like slaves to enlarge Brand Hex' reach. By the time Noh-Varr fought it, it had already marked our planet for other celestial predators with a giant Brand Hex logo. Since as an idea it was unkillable, untouchable and invisible, it could easily hop from avatar to avatar and evade Noh's attacks, but was defeated when Plex uploaded its corporate secrets and tricks to the Internet, and left Hexus without a way to keep expanding, before being shot down with a cosmic bullet.
• The Other. True to its name, The Other is an Eldritch Abomination that is so other that it cannot be perceived by even the most powerful abstracts in existence; it is also incapable of manifesting its true form, provided that it actually has one, in our reality. It must instead use projections of one or more physical entities such as The Greys or a colossal human hand with two large starry eyes on its palm that may or may not actually exist in order to interact with our reality.
• In the Realm Of Kings one-shot, Quasar visited the Cancerverse — an Alternate Universe which was apparently sold to the Many-Angled Ones. Not only does everybody in this world carry an Eldritch Abomination inside their body, but the whole UNIVERSE is also one fricking Eldritch Abomination. It turns out that the whole thing started when somebody who turned out to be Captain Marvel killed Death in this universe, which caused life to grow unfettered... and turn cancerous.

  Quasar: I see it now! I see it all! Their universe! Disgusting! An abomination! Pulsating with corrupt life! One giant, twisted organic mass! Their whole universe is a deathless corpse! A cancer trying to metastasize into our reality! I asked: what’s the worst that could happen? This is. How can I protect the universe from another universe? Galaxies and worlds all united to consume us?

  • The Thanos Imperative event is about said universe finally flooding into the 616 proper. Just to give you an idea of how terrible this is, not only has Galactus joined the resistance, not only have the freakin' Celestials joined the fight, but the Guardians of the Galaxy are forced to recruit THANOS to have any hope of winning in a universe without death. It's especially unnerving since this essentially makes Life itself an Eldritch Abomination. When one thinks about it, it's an invasive species writ large (without death, there was no checks on the growth of life; without checks, life began to absorb everything into itself...).
  • And best of all: you know Shuma-Gorath, mentioned above? Word of God states he's a Many-Angled One, ie, a member of the Dark Gods that rule the Cancerverse. At least it all exploded when Death came back into existence... though given Shuma-Gorath, it's very likely the Many-Angled Ones are okay with that.
• From The Incredible Hercules event Chaos War - Amatsu-Mikaboshi, CHAOS KING. Like Galactus, he comes from a Universe that was before the current Marvel Universe — or rather, he was that universe. He's Anti-Eternity, Evil Twin of Eternity, Anthropomorphic Personification of the Universe itself, an ultimate force of destruction that will not rest until the Universe is destroyed and nothing aside from him remains. And he can squish All-Father level gods like they were flies.
  • The Chaos King's army is composed of deceased gods he murdered and recreated as his puppets. But one of them is Glory◊, a gigantic monstrosity composed of thousands of alien pantheons it assimilated into itself. Its nature is so alien and bizarre that even The Mighty Thor was scared beyond words during their encounter. It's an Eldritch Abomination even to gods. It's also one of the few beings that joined the Chaos King willingly, because it enjoys all the destruction he causes. The above-mentioned Galactus not only joins the fight against the Chaos King, he even says this◊.
  • In The Mighty Thor Annual #1, it is revealed that the Chaos King is actually just one tiny aspect of an infinitely older and more powerful Eldritch Abomination: the abstract known as OBLIVION.
• There are also The Infinites, a trio of immense beings who traveled the multiverse realigning the energies of universes in a way they considered harmonious. They were so big that one of their hands was larger than a galaxy. It took The Avengers summoning Eternity, the living embodiment of the Marvel Universe, to face them. When they learned that their actions were causing the deaths of countless beings, they were horrified and abandoned their experiment and began to make amends throughout the universes they visited.
• Thanos and Galactus also once ran into a being called the Hunger, which operated on a universal scale. It was to Galactus what Galactus was to human beings. It was depicted as a gigantic black void with glowing eyes of fire, because nothing else could portray a pit where existence ceases. Thanos managed to prevent his arrival in this universe, and cut off a piece of the being, which withered and died. But its main body is still out there.
• The Phoenix Force is a cosmic entity that embodies both creation and destruction, manifested as a bird of prey made from cosmic flames. It is powerful enough to destroy the universe itself, and seeks out hosts with psychic powers. While not strictly benevolent or malevolent, its Dark Phoenix aspect — as shown in Avengers vs. X-Men — has a penchant for eating stars and planets to maintain its energy. This was particularly displayed when Thor managed to hurt it in deep space, smashing it halfway across a solar system. This turned out to be a case of Nice Job Breaking It, Hero, as the Phoenix promptly ate the nearest planet. The Worf Effect ensued. When Jean Grey — or rather, the Phoenix Force impersonating her — became the Dark Phoenix in The Dark Phoenix Saga, it caused a planetary-level genocide and nearly inspired an interstellar war.
• Robert Reynolds, aka The Sentry, is implied to only be as human as he believes himself to be. When he reappears after being atomized by Morgan le Fay, the Dark Avengers - a group including the likes of Bullseye and Venom - are freaked out. And then there's the fact that his Superpowered Evil Side, the Void◊, has been around since Biblical times as the Angel of Death and is powerful enough to destroy planets, if not galaxies.
• And from Young Avengers vol.2 we have the Big Bad, Mother - a parasite from another dimension, who feeds on powers of reality-warping children (only this kind of power can hurt it, but kids aren't experienced enough to consist a challenge for it). It's abilities allow some really freaky transformation, as well as power to warp reality around it, taking control of adults, obscuring themselves from the eyes of those who are strong enough to resist mind control, and transforming things in bizarrely creative ways with a bit of meta touch (it once trapped Billy into an empty panel frame). It's home dimension is an Eldritch Location on itself. It's currently using the form of Teddy's dead mother. Oh, and at one point it eats the narrator, while he begs readers to save him.
  • Patriot Costume may be this - nobody knows where it came from, why it took the form it did, why it kidnapped Speed and what it wants from the Young Avengers. However, it can freely walk in and out of Mother's home dimension and not get attacked. It seems to even creep out Mother a bit.
• Venom Vol. 4 reveals that the symbiotes are formed from a fount of eldritch darkness called the living abyss, created by the dark god Knull — whose own existence predates the universe — in order to devour the gods and their creations.
• It's safe to say that the Marvel Universe is basically a Cosmic Horror Universe. Heck, at one point Marvel published Conan the Barbarian comics which were considered to the be part of the Marvel Universe... and the Conan franchise is generally thought to be part of the Cthulhu Mythos.
• Immortal Hulk introduces the "One Below All", the entity that resides in the deepest layer of hell, who serves as the direct Evil Counterpart to the One Above All, aka God in the Marvel Universe. That prospect alone makes the likes of Shuma-Gorath and Dormammu look like Adorable Abominations by comparison, but what's even more frightening is that when it possessed the Hulk, Doctor Strange asked Mephisto (a literal Satanic Archetype) to remove it from him and Mephisto remarked that he couldn't as this being was beyond even him, the ruler of hell. The One Below All is also connected to Gamma Energy and manipulates and controls many of the Gamma Beasts Hulk and co face. Fortunately, while it can influence the multiverse beyond its domain, it cannot directly affect it - it needs a host, who becomes both themselves and the One Below All. Not that this is much comfort; a flash-forward has shown that if the One Below All possesses the Hulk, the combined being will outlive the current incarnation of the Marvel universe (Galactus-style) and exterminate all life in the next one.