One Below All

The One Below All is a mysterious and powerful demonic entity introduced in the modern Marvel comics era as the main over-arcing enemy of the entire Immortal Hulk series, though by the nature of the work the One Below All is being set up as a crisis-level threat that will somehow endanger the entire Marvel multiverse, though for now the entity is very enigmatic and only mentioned briefly and encountered by a few select characters in the arc - which is still on-going.

History
the events of this arc are still on-going, for sake of both simplicity and as a safeguard against unnecessary spoilers the history section of this page will be updated as the arc gains more solid evidence to the entity's purpose and origins.

Appearance
so far the One Below All has no set "true form" but has very brief appearances where it can take seemingly any form it desires - though it favors the use of extremely abstract and terrifying visages, no doubt to further play on its dreaded reputation as the possible "true" Satan of the Marvel multiverse.

Abilities
The One Below All is so far considered an all-powerful demonic entity of unparalleled might, even Mephisto (often considered the closest Marvel has to a Devil figure) stated that the One Below All is far beyond his power and that even if he wished to act against the entity he would fail : this ranks the One Below All as easily one of the most powerful entities in Marvel's history.

Trivia

 * The One Below All is one of the most mysterious and powerful demons to ever arrive in the Marvel multiverse, due to its immense power and its name many suspect this entity may be the evil-half of the One Above All - or an equally powerful (but evil) deity.
 * The One Below All shares much in common with DC's own demonic super-entity known as the Great Evil Beast, it also seems to share a lot in common with Barbatos from the Dark Multiverse.
 * there are numerous incidents within the series that suggest Hulk himself is either the host for the One Above All or may even be the entity itself.