Synopsis
In Darkhold Alpha Dr Doom discovered the original flesh-bound Darkhold and read in it via Doombot, awakening Chthon imprisoned in Other-Realm. Doom entered Other-Realm but returned in defeat. Scarlet Witch read of 5 Darkhold Defenders who in ancient days drove Chthon back to Other-Realm, and she summoned 5 modern equivalents Black Bolt, Blade, Iron Man, Spider-Man (Peter Parker) and Wasp (Janet Van Dyne). They each read a little bit of the book to prepare themselves to face the demon, but they read too much and were transformed into evil versions the Darkhold Defiled.
Since then we've had 5 1-shots showing the alternate lives that turned the heroes evil.
Now in Other-Realm Chthon's demonic army stands around a hole into Earth's reality. They include 5 large figures whose armour marks them as the Darkhold Defenders. The Elder God prepares to lead them through.
Meanwhile in Abysmia somewhere below the USA Scarlet Witch and Doom's right-hand woman Victorious face the 5 Darkhold Defiled. Wanda Maximoff throws the book to Zora Vukavic telling her to take it to her master. Then she binds the 5 in the pink Bands Of Belasco (makes a change from the Crimson Bands Of Cyttorak) and sends them through the portal to Other-Realm.
Over there someone watches events (as he did in Alpha). The 5 Defiled arrive and Iron Man suggests they kill Chthon and take over the Realm. They think they're standing on the remains of the 5 Defenders. But these remains rise up to fight them as the armoured giants we saw earlier. However the old 5 are easily defeated by the new 5, especially by Wasp who takes command and leads the others to slaughter Chthon's army. In the midst of the battle Black Bolt speaks and nearly destroys his allies along with a large proportion of the foe. Then IM unleashes nanotech which chops up a lot more demons.
When the fight is over Spider-Man calls on Chthon to show himself. Which he does as a really big giant who holds all 5 of them in the palm of his hand. Spidey stops Blackagar Boltagon from dangerously speaking again as Wasp goes giant-size but she can only stop the hand from closing on them. Then the mystery figure joins them in a ball of light. He emits a blast which forces Chthon's hand open, at the same time shredding his own cloak to reveal a superhero costume. Iron Man detects he's from the real universe. Blade tries to kill him as unwanted competition but a smaller blast repels him. Then Chthon appears in full, but only slightly taller than the humans. He disregards the Observer as temporarily out of power. And he tries step through the hole to Earth but something stops him ...
... and Dr Doom (with the Darkhold) and the Witch stand in his way. They trade insults and then Chthon's head turns tentacled monster. The duo combine spell power against him. Wanda admits that neither Chthon nor the original book can be destroyed, but Victor Von Doom says he will use the Darkhold to bind the demon. However SW zaps him and takes the book herself. After all the times Chthon has tried to use her as his vessel on Earth, *she* deserves revenge. And she magically absorbs the book into herself. And then she has the power and strength to absorb him too.
The 5 Defiled start to attack her but she warns them that if they kill her it will free Chthon again. Then she sends them back to Abysmia and they are reverted to their goodie versions anyway. She tells them that she scrubbed their evil lives from their memories except, at the behest of Black Bolt, a few shreds to remind them of what they *could* have been. Then Wasp detects that someone else escaped Other-Realm with them ...
... and she flies to Pennsylvania/Appalachia where the mystery hero is standing in the middle of a street with a female head floating near him. The head says that the man is Model X3Z/Omega The Unknown, her son James-Michael Starling (and we see omega-marks on his palms). Long ago he went on a dimensional pilgrimage but got trapped in Other-Realm. While his designated mother head waited for him to return she's been compiling data about Earth.
Our last epilogue is in Dr Doom's long-abandoned castle in upstate New York where he and Scarlet Witch have a tense meeting. They effectively agree to be enemies. Then Wanda zaps Victor with a 1-finger spell and he turns out to be be merely a remote-controlled Doombot. SW flies away claiming to be free of her past. And we're promised more of her soon.