In the original Infinity Gauntlet saga, Thanos after he snapped half of all existence from reality. Marvel's Devil helped the Mad Titan acquire the massive cosmic power of the Infinity Stones for his own personal gain, but ultimately, Mephisto was responsible for giving Marvel's heroes the chance to take down the supervillain.
In the original Infinity Gauntlet arc which ultimately inspired Thanos' quest in the Marvel Cinematic Universe, the Mad Titan wanted to snap half of reality out of existence to prove to the living embodiment of death that he was worthy of her love. In Avengers: Infinity War and Avengers: Endgame, Thanos' motivation was entirely different, as Death wasn't involved at all and he sought to get rid of half of life in order to create balance in the universe. Mephisto never appeared in the MCU, although Avengers directors Joe and Anthony Russo have said that Ebony Maw from Thanos' Black Order plays a similar role as him in the comics.
In the epic Infinity Gauntlet storyline from Jim Starlin, George Perez, and Ron Lim, Mephisto plays a key role in Thanos' using the Infinity Gauntlet against the Avengers. Marvel's Devil encourages Thanos to use the powerful weapon in order to rewrite reality while intending on taking it from him for himself afterward. At the height of the battle with the heroes on the ropes, Mephisto convinces Thanos to give them a small, meager chance at victory, as it will impress Death since their current odds were at zero percent. Thanos buys the trick and agrees to fight the heroes with just the Power Stone. He gets rid of a guaranteed victory based on Mephisto's advice.
Even Eros, Thanos' brother whose ability to talk was taken away from the Mad Titan, spells out Mephisto's move: "it was most subtly done, but the Devil just save the lives of Earth's heroes." While Thanos wouldn't be defeated immediately (and was eventually betrayed by Mephisto), allowing the heroes a small chance at winning helped propel them to victory.
The moment feels quite similar to when Doctor Strange looked at all the future realities and determined there was only one (in fourteen million) way to win against Thanos in Avengers: Infinity War. In this case, the Avengers had no shot at winning until Mephisto convinced Thanos to take his God-like powers down a notch in order to impress Death. It wasn't easy, but Marvel's Devil afforded the heroes a small chance to win against Thanos, which they eventually did. Strangely enough, the Mad Titan's defeat would have not been possible without Mephisto.
