The Archenemy was a powerful magical creation and major antagonist in the Marvel universe - it lived to destroy and consume all magical energy, making it a truly terrible opponent to mystical beings of all shapes and sizes.
Shortly after taking over the role of ruler of Otherplace Magik had her Earthly aide, Nugent, create a computer program/spell that began to assemble, catalog, and codify a database of arcane lore from all sources. Magik intended this to be a powerful weapon in her ongoing war, a research engine she could rely on, based in the mortal plane, untainted by the background magics that permeate the splinter realms.
Nugent relocated his program from his lounge to the bathroom, so that his bathtub could drain away the secretions the program accumulated.
The repository of magical knowledge evolved, drawing together all the vast elements of eldritch data within it and becoming...something else. From Nugent's machine sprang the Archenemy, an absolute force of magical evil. It accumulated more and more power over an undisclosed time and then returned from the far future to conquer and safeguard the time of its birth.
S'ym somehow became allied with the Archenemy, which allegedly promised S'ym that he would rule over Otherplace after the Archenemy had destroyed the other Splinter Realms. S'ym took the form and identity of Duke Bleys, becoming one of Magik's chief advisors.
The Archenemy appeared in the skies of the Nightmare Realm, slaughtering the Silurian Guard and overrunning the fortress of Nightmare.
The Archenemy caused a horrific nightmare in Nightmare himself before confronting the demon directly.
The Archenemy apparently consumed both Nightmare and much of his realm, adding it to its own power.
Some demons from Nightmare's realm escaped the realm's destruction.
The Dark Dimension of Dormammu fell under the Archenemy's onslaught.
Dormammu, sensing the above conflict in the Nightmare realm, traveled there and discovered Gleeg, the decapitated former captain of the Silurian Guard. Dormammu reanimated Gleeg and forced him to reveal what had happened.
Nugent showed his work to Magik, who was pleased with his progress. Nugent prepared to transcribe the Book of Eibon into the database.
The Archenemy appeared in Muspelheim, annihilating Surtur's demon guards, extinguishing the eternal flame on Surtur's blade, possibly destroying Surtur as well, and leaving the realm a burnt out husk.
Demons from Nightmare's realm assaulted the fortified wall of Otherplace before being fought off by Magik and her forces, including Vitchen and assisted by Nightcrawler (Kurt Wagner). Dormammu then appeared in Otherplace, confronting Magik.
Dormammu forced Gleeg to tell his story to Magik before allowing Gleeg release from the agony of life-death. Dormammu proposed an alliance against their unnamed foe, though Magik was hesitant to trust the demon-lord. Wishing proof, Magik and her armies accompanied Dormammu to the realm of Muspelheim. After surveying the destruction, they encountered the Archenemy itself, which slaughtered Magik's warriors and Dormammu's Mindless Ones alike. Magik used a stepping disk to transport herself, Dormammu, Nightcrawler, and Vitchen back to Otherplace, escaping the destruction that the others faced.
Magik checked in on Nugent, who reported that his machine was already stocked with three-quarters of the world's arcane data. She instructed Nugent to keep at it, as she feared she would have need of the database soon. Nugent tabulated the information Magik gave her and predicted that the next target would be Hades (more correctly, Mephisto's realm of Hell). Magik then traveled to Hell and confronted Mephisto, forcing him to listen to what she had to say. Magik then showed him that the Archenemy was at the gates of Hell itself.
As the Archenemy lay waste to Mephisto's beast legions and kingdom, Mephisto struck out at the Archenemy, attempting to destroy it and having virtually no effect on the entity. With no other choice, Mephisto accompanied Magik back to the Otherplace, escaping Hell before it fell under the Archenemy's assault. Deciding that only a united force could oppose the Archenemy, Magik called a convocation of the Lords of the Splinter Realms. After much debating, the Lords agreed that they had no choice but to put their trust in each other, and they made a pact of union, merging the Splinter-Realms into a single base from which they could act via a collective showering of energy, mingling of oaths, and mixing of demon-blood.
However, almost immediately after they had done so, Magik's advisor Duke Bleys was revealed as her enemy S'ym, who was serving the Archenemy. As the Archenemy itself arrived at the merged realm, S'ym revealed that they had actually made the Archenemy's job easier by creating a single easy target.
Magik blew the Horn of Harrowing, calling the various Lords and their servants to confront the Archenemy, who paused briefly—as if stayed by the sight of such formidable resistance—before beginning its onslaught. Two hundred thousand died in the first second, and the Archenemy moved on, crushing the walls of the United Realm underfoot. As her allies fell before the Archenemy, Magik convinced them to unite their power once more and focus them through her so that she could cast a spell of divination to understand the nature of their foe. As soon as she understood what it was, she transported Nightcrawler to Manhattan, where he destroyed Nugent's machine. Weakened by the destruction of its source of power, the Archenemy fell before Magik and her Soulsword, which destroyed it.
With the Archenemy destroyed, the United Realm swiftly splintered anew. Many of the demon-lords were able to re-form over time.