Eternity is an abstract within the Marvel cosmological hierarchy. He is the embodiment of the Marvel Universe as an M-body and is the embodiment of the Marvel Multiverse in his full form. The distinction between Eternity M-body and Multi-Eternity isn't always cleared defined but it does explain some of his jobbing habits and the apparent difference in power between those he interacts with.
1) Powers and Abilities
He can do anything he pleases though that is only limited to the Multiverse inside of him. Sometimes he chooses not to interfere in some matters though that doesn't make sense at times.
He knows everything that goes on in the Multiverse inside of him. Everything else outside is a mystery.
He is the Marvel Multiverse but isn't everywhere at all times or occupies everything. There were beings older than him and beings that exist outside of him.
All characters have a beginning and Eternity is no different despite his name.
2.1 First Cosmos
In the beginning there was no Marvel Multiverse but rather a singular Marvel Universe. This is the first cosmos known as the First Firmament.
2.2 Ancestor of Eternity
The First Firmament is the ancestor of Eternity and is much bigger too.
2.3 Celestials and Aspirants
He is responsible for the creation of Celestials and Aspirants.
2.4 Second Cosmos
When the first cosmos collapsed, a new second cosmos took its place and gave birth to a new Marvel Multiverse.
2.5 Family Lineage
This would continue to be the case for 7 more times to come where 7 different cosmos were born from the First Firmament.
3) Eternity (M-Body)
M-bodies or manifestation bodies are frequently used by Eternity. These are projections of his power where most of low showings can be attributed to.
3.1 M-body Evidence
Eternity describes himself and other abstracts as using M-bodies due to not having any true physical forms to physically interact with.
3.2 Universal M-body
The M-body of Eternity refers to himself as the Marvel Universe.
3.3 Universal M-body
The M-body of Eternity refers to himself as the universe from which Thanos wishes to usurp.
3.4 Fight vs the Infinity Gauntlet
The M-body of Eternity puts up a better fight against Thanos with the Infinity Gauntlet than all the other abstracts even though he eventually loses too. Notice this Eternity refers to himself as universal and has done so previously.
3.5 Universal Control
Shows and explains to everyone in the universe why Galactus is important.
The M-body of Eternity and the M-body of Kismet are described as living universes.
4) Multi-Eternity (Seventh Cosmos)
4.1 Multi-Eternity is the Multiverse
There is a clear difference between Eternity as an M-body and Eternity as a whole or rather what he should be referred to as Multi-Eternity.
4.2 Multiversal Multi-Eternity
Eternity in his true form compasses the whole Marvel Multiverse
4.3 Children of Eternity
Eternity in his true form is responsible for spawning the 6 friendless who are his children.
4.4 Importance of Eternity
Eternity in his true form is one of the faces of the Living Tribunal that represents Neccesity.
4.5 Multiversal Multi-Eternity
Dormammu refers to Eternity in his true form as the most powerful being in the Marvel Multiverse before proceeding to use that power to remake the multiverse in his image.
4.6 Fundamental Multi-Eternity
Doctor Strange expresses concern over Dormammu absorbing of fraction of Multi-Eternity's power and states that Eternity in his true form is the embodiment of everyone and everything.
Dormammu with Eternity's power is described as omnipotent, omniscient and omnipresent. This is clearly false due to the existence of the Living Tribunal and the One Above All but it does speak to the sheer power of Multi-Eternity.
4.8 Multiversal Control
Since he is the Marvel Multiverse, Multi-Eternity has full control over himself and everything that goes on inside of him unless the plot says otherwise.
4.9 Cosmic Hierarchy
Eternity is ranked equal to Infinity and above the likes of Master Order and Lord Chaos but below the Living Tribunal.
5) Multi-Eternity (Eighth Cosmos)
To be updated
6) Confusion over appearances
Not every version of Eternity is the same just like not every version of the Phoenix is the same or every version of the Spectre is the same. Characters whose power is of a multiversal magnitude are usually too powerful to interact within a singular universe thus they usually opt for lesser and more convenient forms such as the DC Presence who is omnipresent in his true form but mostly adopts the appearance of a dog when within the mortal world.
6.1 Eternity vs Chthon
Eternity was apparently being killed by Chthon which wouldn't make sense if this version of Eternity is in his true form. If this version of Eternity is in his true form then does that mean Chthon is of multiversal power?
6.2 Eternity vs Dormammu
Eternity has had multiple run-ins with Dormammu and these fights seem to be closer than it should. The best way to explain this is that Dormammu fought with an M-body of Eternity.
When faced against the true form of Eternity, Dormammu was seen as insignificant compared to him.
6.3 Eternity vs Marquis of Death
During an inter-dimensional war between the past and future versions of Clyde Wyncham, Eternity was summoned into battle by the future Marquis of Death. This feat is often used to establish the Marquis of Death as multiversal but then again the battle wasn't as one-sided as it should've been and ultimately it weakened the Marquis of Death. If Multi-Eternity was called into battle, then it does raise more questions than answers.
6.4 Eternity vs Heart of the Universe
Eternity along with Infinity and the Living Tribunal were absorbed by Thanos with the Heart of the Universe yet the Marvel Multiverse wasn't destroyed. The most likely explanation is that these abstracts were using M-bodies.
Eternity as an M-body is universal of the highest order but his power can be superseded by the Infinity Gauntlet which has nigh-omnipotent power but universal range. Every time he seemingly jobs can be attributed to this form otherwise the embodiment of the Marvel Multiverse jobbing should result in a series of cataclysmic events yet the Marvel Multiverse still survives.
Multi-Eternity or Eternity in his true form is the Marvel Multiverse or the overall cosmos. Nothing that was born from within him should be stronger than him. This includes the likes of Dormammu, Chthon, Franklin Richards, the Infinity Gauntlet, the Heart of the Universe and the Marquis of Death. Not every single appearance of Eternity is Eternity in his true form which is the same for many fundamental godly beings that serve a purpose to all existence. Multi-Eternity is hardline multiversal and no higher.
Low End Power Level - Solid Universal (as M-Body)
High End Power Level - Solid Multiversal (in full form)