Synopsis
Edwin Jarvis is tidying the area of Avengers Mansion grounds which contains statues of Avengers when a costumed young man appears in a blast of light and says he's Tyfon from the future. He's come back to this day to warn Jarvis that the Avengers are fated to die this day, and only Edwin can save them. Jarvis says the Avengers are in Manhattan dealing with a problem.
We see the villainous Black Knight brought down by Scarlet Witch, with Quicksilver dealing with falling rubble. Meanwhile Pietro's wife Crystal faces Melter (who she correctly thinks is dead) and the very-much-alive Radioactive Man. (In fact BK is dead too.) (Notably these 3 were the 1st members of the Masters Of Evil (led by Baron Zemo), and are wearing the costumes they had then.) Crystal stops Melter's attack with ice, and Vision pops up to do his hand-in-the-chest trick on RM. As Vizh examines the unconscious bodies they turn to shadow-matter. Crystal points out that nearby buildings are being replaced by shadow-matter too. Wanda Maximoff contacts Captain America to tell him what's going.
Cap is with Iron Man (Teen Tony) fighting Living Laser. When he threatens bystanders Hawkeye hits him with a (blunt) arrow to the head and then swings in to deliver a 2-footed kick which shatters the foe into shadow-matter.
Meanwhile Tyfon is explaining to Jarvis that history tells him that the Avengers' greatest enemy returned today and opened a portal to a dark dimension, letting in a darkness that overwhelmed the world for generations. The Avengers all died in the conflict. Their records don't say who the foe was, but that Jarvis was a witness. And he hands Edwin something (a camera to record evidence for the future?). He wants Jarvis to review Avengers history in their files so he can read the associated memories from his mind. The enemy is recreating old foes so there may be clues in what Jarvis knows. Edwin asks if he should start from the beginning, but Tyfon tells him to start later where records are skimpier. And he seems particularly interested in Thor ...
... who we find with Giant-Man, Wasp and Black Widow apparently reliving the Kree-Skrull War. But these guys also turn into shadow-blobs. Then an enormous Kang/Rama-Tut/Scarlet Centurion/Immortus hybrid taps Henry Pym on the shoulder, reverting him through *his* previous super-ids. But Thor's hammer Mjolnir smashes the fake time-lord.
The other Avengers join them in time to be attacked by Dr Spectrum, Hyperion and Whizzer of the Squadron Sinister. Hawkeye, Black Widow and Quicksilver are knocked out, and Cap tells Crystal to get them to hospital while the other Avengers continue the fight. But the 3 baddies are replaced by a horde of Lava Men.
Back at the Mansion Tyfon and Jarvis are looking at Avengers' foes including Thanos, Arkon, Stranger(?), Grey Gargoyle, Enchantress and Absorbing Man. Jarvis would still find it simpler to cover things in order from the start, but Tyfon says they've got to search faster.
The Lava Men presumably went the same way as all the others, because we now find Iron Man fighting Ultron, Wasp vs Controller, Scarlet Witch and Vision against Grim Reaper while Captain America is attacked by the Assassin from #145-146 (winning this issue's obscure villain prize), Thor faces Count Nefaria and Giant-Man is battered by a Kree Sentry, Nuklo and a Growing Man.
The darkness has spread. Spider-Man (Ben Reilly) and Invisible Woman/Mr Fantastic/Thing of the Fantastic Four try to deal with it in Manhattan, But further afield we see Black Panther in Wakanda, Sub-Mariner in the ocean and She-Hulk in the Mid-West all fighting various manifestations.
As the Avengers start to fail Jarvis is still trying to get back to their 1st villain in their 1st adventure. And Tyfon distracts him with later contenders such as Baron Zemo, Collector, Executioner and Black Talon. In fact he gets quite worked up over Edwin's fixation. Jarvis says he quite understands. It was 1 of their easiest cases with a wimp of a villain. The team are embarrassed to have him in their origin. It was that fool Loki ...
... and now incensed Tyfon reveals himself to be that very villain, who wants to brag that he's responsible for what's happening. He's the team's greatest foe, and he's been using Edwin's knowledge, transmitted by the little demon impersonating the camera, that has created the distracting enemies that have kept them from fighting the darkness that's engulfing the world. Jarvis, who obviously goaded Loki into this reaction, initiates a red alert on his Avengers Identicard.
All the Avengers (except the 4 in hospital, counting Crystal) abandon their fights and rush to the Mansion. It turns out Loki himself is the gateway that the darkness is pouring through. The Avengers attack, and Loki responds by animating their adamantium-coated statues. The team fight a defensive action while Scarlet Witch works up a hex to shatter them (she's neutralised adamantium before). However this time her unpredictable power melts the metal instead, into a wave that threatens to engulf them. Those with blast capability, plus Thor whirling Mjolnir, try to push the wave back. They succeed and it covers Loki, turning *him* into a statue.
After it's all over Crystal calls to say the other 3 Avengers are OK, and in fact Quicksilver is coming to join them. The portal to the Dark Dimension is sealed and the world is saved. Then Cap and Wasp answer the doorbell. It's the mutant called X-Man/Nate Grey who's here to tell them that the world is in danger for another reason, Prof X has gone mad.