Synopsis
In the last Age Of Khonshu arc the Egyptian Moon God foresaw Mephisto ruling the world (or the universe?) and got his servant Moon Knight to steal the powers of certain Avengers (and others) in order to oppose him. They killed the devil and many of his alternates from other realities who came seeking revenge. But then Marc Spector temporarily gained the power of the Phoenix and helped the Avengers take down his boss.
However this issue starts in the 19th Century in the Kansas Territory with a single covered wagon stranded in the snow at the crossing of 2 trails because the mule pulling it has died. Ambrose Randolph Winthrop and his family can't even eat the mule because it is strangely already full of maggots. Winthrop prays for his family to survive and thrive, and his prayer is heard by a snake with a knife. The man makes a deal with the devil at the crossroads, but the price is the deaths of his family. He will survive and marry again, and *that* family will thrive. Mephisto welcomes Winthrop to his army of maggots.
Now in New York Black Panther, leader of the Avengers, confronts Moon Knight. In the background we see cranes dismantling Khonshu's pyramid and repairing damaged buildings. We learn that Thor has taken the Moon God to an Asgardian dungeon, and most of the Avengers think Marc Spector should be in jail in Wakanda for weakening the team when they are beset by other enemies (Dracula, Sub-Mariner, the Squadron Supreme and the Russian Winter Guard from previous issues). MK points out that death won't stop Mephisto so they should be more worried about *him*. T'Challa agrees and asks Moon Knight to make amends by becoming an Avenger (again) and helping them foil the devil's plans. But Spector says he took his shot. Now he'll go back to fighting foes from the shadows.
In Avengers Mountain HQ (the body of a dead Celestial at the North Pole) Captain America, Captain Marvel and Iron Man are surveying the bodies of the alternate Mephistos that Khonshu killed. Tony Stark is shaken by the scale of the pandimensional war they're involved in. He had a vision (#31) of this on the way back from battling Mephisto in the Stone Age and knows that the demon has been sowing his maggots throughout time as well as alternity. He claims that ever since then he's felt the maggots under his own skin, which has Carol Danvers fearing for his sanity.
In the past Ambrose Winthrop became an extremely wealthy man on the west coast. When he died Mephisto came to see his widow and demanded and got a finger from each of her children as part-repayment of the Winthrop family debt. Over the years he was repayed in dark favours or fingers and the occasional whole 1st-born.
In Avengers Mountain Ghost Rider and guests Dr Strange and Iron Fist (whose powers were among those stolen by Moon Knight) are watching the baby Starbrand calmly floating in midair. Robbie Reyes worries that this Starbrand knows that he killed her predecessor (in the Legacy 1-shot). Stephen Strange says there have been a series of Starbrands, as well as Iron Fists and Sorceror Supremes, through the centuries going back to the Avengers of 1 Million BC. Danny Rand objects that the Book Of The Iron Fist doesn't record anything that far back. But it does say a lot about visits by the Phoenix Force, most of it bad.
Blade and Thor are watching the Phoenix Force 'nesting' atop a nearby mountain. Blade wants to know how to kill it. He says that She-Hulk (presumably as lawyer Jennifer Walters) is trying to stop the world leaders from bombing it, because that would only make it angry. But he suspects Thor *does* know how to kill it. The current All-Father of Asgard says that the Phoenix Force is here looking for a new host. He says he has experience with it in combat and in 'other' activities. (If this is a hint at a sexual relationship then the only 1 I know of that it could refer to would be with Odin, as suggested in #4.) But he says he *does* know how to make it bleed, but he would probably die in the act.
After this we have a series of short flashbacks, beginning with 4 billion years ago when the diseased Celestial we know as Progenitor (#3) died on Earth and infected it. Mephisto was there as a fly laying his maggots. 1 million years ago he persuaded young Thanos to attack the Avengers 1MBC. In Ancient Egypt he goaded Apocalypse to confront Thor and another band of heroes. Some years ago he fanned Hulk's mistrust of the Avengers (seems like somewhere within the original Av#3). Then far in the future he awakens Dr Doom the Living Planet to face Phoenix-Wolverine and the Goddesses Of Thunder (Thor (2018) #5-6).
Back in the present Howard Stark (resurrected by Mephisto in #31) arrives at the home of Oliver Winthrop, great-grandson of Ambrose. He is accompanied by a Hellhound who kills and eats the whole family. Stark says the debt is now paid in full, and he asks the Hound if this is enough to power the resurrection spell. Mephisto answers from the Hound's body.