The Devil's In The Details
Life for Miles Morales has been hectic as of late. He just played a major role in Spider-Geddon, forming an unlikely respect for Otto Octavius and even serving as a host to Captain Universe. His uncle Aaron returned as the Iron Spider and assembled a new version of the Sinister Six. He dealt with an active shooter situation at his high school, fought against his fate to kill Hydra Captain America and managed to survive Weirdworld. He's been understandably under a lot of stress, especially for a teenager. Not even Peter Parker had all of that thrown at him so early in his superheroic career.
Mephisto offering him something makes sense for the demon, but what could push Miles to the point where he would consider working with Mephisto? Throughout Champions #1, Miles is just... off. He's clearly lost in thought, not celebrating the success of the team or even speaking with his friends. While his short attitude with Sam Alexander comes off as just sheer stress, his anger with Ms. Marvel feels more furious. He's clearly not upset with her, but he's upset all the same. This supports the theory that he has struck a deal he can't escape. The supposed bargain might even have something to do with the expanding roster and strength of the Champions, which could be what's terrifying Miles so much.
All of this ties into a single, strange page in the book. The first chunk of the story follows the Champions, split into three teams, dealing with three threats. But during a montage of the heroes uniting around the globe, there's a single page of a dark and violent vision. Miles watches on, his mask damaged, as many of the other founding members of the Champions scream out in pain while surrounded by destruction.
It comes suddenly, and isn't commented on at all in the scene. But with the revelation that Mephisto's involved, there's no way it ends well for the heroes. What kind of deal did Miles make with Mephisto, and what for? Is this "One More Day" act going to be responsible for the return of someone he loved, or something worse? Is he responsible for the spread of the Champions through magical means? And, if so, what will the fallout be? Because if that vision really is of the future, then the future looks extremely dour for the young heroes -- and it might all be Miles' fault for making the same mistake his predecessor made.