Apocalypse invades Atlantis and Doctor Doom offers the Atlanteans a formula which mutates them in a way that allows them to breathe upon the surface, thus allowing them to escape from Apocalypse. Unfortunately, the mutation caused by the formula had some side effects, driving a lot of the Atlanteans who consumed it insane, thus leading them to terrorise humans, who confuse them as normal mutants, so they grow mistrustful of the mutant race and put the cause of the X-Men in jeopardy. After struggling to defeat the Atlantean mutates, the X-Men learn that Doctor Doom was behind this malicious conspiracy, so they invade the laboratory of his castle in Latveria where he produced his despicable formula, and he claims that he intends to send the X-Men to Mephisto's realm through a portal he revealed to them, while displaying another portal supposedly for his escape, so the X-Men throw him to the portal heading to Mephisto's realm while they used the other portal to find Apocalypse, correctly assessing that Apocalypse invaded Atlantis so that Doctor Doom would find the opportunity to offer his formula to some Atlanteans brought into despair by Apocalypse, as he had already been working with Doctor Doom in the first place. Surprisingly, Doctor Doom was bluffing, and in fact he actually wanted to go to Mephisto's realm while the X-Men went to Shi'ar territory through the other portal, trapped away from the galaxy while not knowing if they had managed to end humanity's suspicion of them due to the Atlantean incident on Earth. Therefore, the negative emotions of the X-Men, alongside a conflict with the Shi'ar due to another coup staged by Deathbird against Lilandra, allowed Onslaught to return after the defeat of the X-Men and kill both Lilandra and Deathbird, thus allowing Gladiator to emerge as the Emperor of the Shi'ar. The enormous power of the Shi'ar Empire proves too much for even Onslaught to handle, but just before his imminent defeat, he manages to do what the X-Men couldn't, opening a portal for his return to Earth. Doctor Doom wanted to go to Mephisto to inform him about his contribution to the deaths of billions of Shi'ar citizens that have empowered his realm, in order to earn his support in his endeavor out of gratitude. Back on Earth, Namor, who had encouraged a significant portion of his Atlantean people to deny Doctor Doom's formula, is fighting Doctor Doom, Apocalypse and Mephisto alongside Sentry, who is one of the few superheroes of Earth to survive and has also managed to form a coherent connection to the Void due to the magnitude of the threat they are both facing, and Ultron, who is entrusted by humanity, ineffective in its efforts to protect itself, with the command of a massive mechanical army. Gladiator, realising that he is an incapable ruler of the Shi'ar, leaves the governance of his Empire to other hands, but not before he assumes leadership of an expedition force to invade Earth for causing all this destruction to his people. While the armies of Gladiator and Mephisto on one side and the Atlanteans alongside Ultron's robots on the other side are fighting in an epic war dwarfing World War 2 in scale, Doctor Doom, Gladiator, Apocalypse and Mephisto are confronting Namor, Sentry, Onslaught and Ultron in Paris with no prep, no intervention and standard equipment and powers, until all heroes from either team are dead.

I don't get these matchups.

Mephisto stomps a hole in everyone here.

The power scale is so wild in these threads but Mephisto solos.