Zom is an incredibly powerful monster from Marvel comics and an enemy of Doctor Strange, Zom is powerful enough to dwarf Doctor Strange and beings such as Umar or Dormammu with ease - however he is nowhere near as powerful as the Living Tribunal.
Zom claims to have been created to destroy, but his creator and the methods of his creation are unknown. Ages ago, he was defeated under unknown circumstances, which may have involved a union of several other extremely powerful beings, including Dormammu (the ruler of the Dark Dimension) and Eternity (the cosmic being representing the entire temporal axis of the Earth dimension). To protect the universe from his might, Zom's hands were bound with Chains of Living Bondage by Dormammu. Zom was forced to wear a Crown of Blindness, and his essence was imprisoned by Eternity within an Amphora in a "world beyond all worlds" in a "time beyond all time."
However, when Umar - Dormammu's sister, moreby cunning and cruel than her brother, her power bound only by her less-powerful realm - came to Earth to confront Doctor Strange, casually slicing through the entire planet to get to him, the wizard travelled to the distant realm and unleashed Zom on Earth; he correctly surmised that Zom would attack the most powerful being in his vicinity and that was Umar. The fearsome Umar's fury melted like snow in the sight of the demon, and she fled back to the Dark Dimension, vowing never to return.
Now, however, Strange had to contend with Zom himself; as the monster despatched the Ancient One, Strange's teacher and the then-Sorceror Supreme, the old wizard's final words gave him the knowledge he needed to defeat it, however. Defying Zom's power, Doctor Strange severed Zom's forelock, a tuft of hair sprouting from the monster's brow, filling the giant with panic and sending a wave of power across the entire planet. Soon, the Earth trembled as the Living Tribunal, the arbiter of all reality, appeared in person and immediately annihilated Zom. The Living Tribunal then chided Doctor Strange for unleashing the monster, and for releasing great evil into the world by cutting his forelock; apparently the appendage acted as a mystic shield, not only hiding its own presence but, by extension, knowledge of evil mystic powers in general, which were now stirred awake in people across the planet. Zom himself, however, had been defeated.
Zom's recently returned during World War Hulk when a desperate Doctor Strange utilized Zom's power to take on the Hulk, but became overwhelmed by the bloodthirsty demon and had to work together with several other heroes to stop Zom from worsening the situation.
While defeated and eventually imprisoned, Zom's essence remained imprinted on Strange's soul and repeatedly manifested again, until, during the universal assault of the Chaos King, Marlo Chandler used her Death-related powers to annihilate the Zom remnant inside Doctor Strange for good.
POWERS AND ABILITIES
Zom possessed absolutely top-level magic powers, demonstrated by his ability to handily defeat the Sorceror Supreme; he not only wielded immense power, but also possessed the knowledge to apply it in complex spells. He was capable, or had been rendered capable, of hiding his existence from the Living Tribunal.
When destroyed, as has happened several times, Zom can reconstitute himself completely from the smallest of remnants left of him, even the tiniest fragments remaining inside the amphora he was originally imprisoned in, so long as he has access to a source of mystical power. He once rebuilt himself from the traces of mystical energy in Doctor Strange's magic repository, although in a severely reduced form; however, when Strange ingested his essence, he manifested completely or almost completely in minutes, since he had direct access to the Sorceror Supreme's own mystical reserves.
When destroyed, Zom has repeatedly 'possessed' the most powerful object or person in his vicinity; examples include the demolished Hulkbuster armor Iron Man used, as well as Amadeus Cho, the Hulk's superintelligent companion. However, in the latter case he possessed no enhanced strength or magic powers, and was easily knocked out and contained once again. Zom appears to be like a computer program, capable of having several versions of him active at any given time; even as his essence was imprisoned in the amphora once again, another version of him existed inside Doctor Strange himself.
Zom is unquestionably one of the most powerful and deadly beings in the Marvel universe; as originally, he was imprisoned by a coalition of beings including Eternity and Dormammu, normally mortal enemies, and when he manifested once again, this provoked a direct intervention by the Living Tribunal, who, on the other hand, did not even interfere when Thanos gathered the Infinity Gems.