Doctor Strange has always found a way to improvise and save the world, even if it goes against the beliefs of the Avengers. The fabric of world isn't all he has to take into consideration, as the multiverse usually falls under his purvey too. And so, with Dormammu being consumed by Galactus, and magic and science dissolving into each other across every realm, the Sorcerer Supreme ends up making a deal he will likely soon forget.
This comes courtesy of Mephisto, providing this particular devil with another ace of his sleeve for an enigmatic larger endgame. We've seen the villain pop up in myriad Marvel titles for months now, and it looks as if this bargain with the Master of the Mystic Arts will provide him with his most powerful bargaining chip yet.
When Galactus becomes too powerful too handle, Strange forces Dormammu's lackey, Ykolon, to send him to Mephisto's lair. After hearing that Mephisto was the one who spurred on Dormammu to unsuccessfully co-opt Galactus, Strange knows he can find answers there. However, he also finds his former lover Clea and Umar, Dormammu's sister, who are held in the lair as hostages. Knowing he needs them to fend off the Devourer of Worlds, Strange agrees to perform one deed for Mephisto when called upon, as long as it doesn't involve the loss of life. As much as that sounds promising, we know how "One More Day" played out when Mephisto brought Aunt May back to life by dissolving Peter Parker's marriage to Mary Jane.
Strange is equally desperate, and the devil salivates over having negotiated this victory. He sets Clea and Umar free, sending them back to fight the gigantic being. When Clea chides Strange for brokering the deal, he admits he has no choice and will confront the cost later, as their only hope is to assemble a legion of cosmic and supernatural heroes from across the Marvel-616 universe.
Avengers #21 depicts Mephisto literally playing Earth's Mightiest Heroes against the Squadron Supreme and their handler Phil Coulson, whom the demon resurrected after the events of Secret Empire. He's even seen moving pieces on a chessboard in Las Vegas, although all we know so far is the government considers the Avengers a threat, so Mephisto may be orchestrating something along the lines of Civil War. Black Panther is already stepping on toes, so the demon's plan -- whatever it may be -- looks to be coming together.
His manipulation goes further back, however, as he also brought members of the Champions back to life in a battle that Miles Morales lost. The new Spider-Man eventually made a deal to bring his friends back, at the cost of one innocent life. Be that as it may, the deal had no price attached, so we're still waiting to see whether Mephisto merely wanted to erode Miles' sanity through guilt, or if he has another angle.
Lastly, Mephisto was also seen stealing Wade Wilson's heart in Deadpool. Ultimately, we have no clue how these threads will be stitched together, but the pawns (Namor, Odin, Starbrand, Moon Knight, Ghost Rider, Black Panther, the Squadron, Malekith and the Russian Guard) have been lined up to set up the ultimate checkmate.