Mephisto was a demon lord ruler of Hell, he was also a member of the Lords of the Splinter Realms, and the Hell-Lords.
Mephisto brought Hellcat to Hell, where she died, was eventually resurrected, and developed mystical abilities after the resurrection.
While the Kingpin was battling the Avengers, and the Hand was helping him, their members were also collecting mystical relics, to use them with the intent of causing a rift in reality so their Demon Lord could join them, they were stopped, but they were still able to cause the rift in reality. Mephisto who was an ally of the Hand's Demon Lord, knew about their objectives, and planned to use the rift himself to travel from his dimension to Earth.
Allying himself with Satana, and Dracula, Mephisto finally decided to go along with his plans to invade Earth. Mephisto materialized part of his dimension close to the Avengers Academy where he planned to launch his attack, Mephisto decided he would take the heroes souls by manipulating them into battling with each other, and destroy them from the inside, eventually in desperation giving up their souls to Mephisto.
After his hordes of demons were defeated, and both of his allies left him when they realized Mephisto was just using them, Mephisto had to face the Avengers himself. Nick Fury gave him the option of going back to Hell and close the dimensional rift, but Mephisto refused that, ended up being defeated by the Avengers, and was imprisoned at one of the Academy's cells.
When Dormammu attacked the Avengers Academy, Mephisto talked to him about a secret plan that the villains were setting up, telling Dormammu that they were just waiting for one person to put it in effect.
He also talked about the plan with Malekith who was aware of it, and was working alongside Mephisto to see it's completion. They mentioned being in contact with Thanos and Hela, possibly indicating that they are also part of their plan.
Avengers Under Siege
Mephisto manipulated Baron Zemo into freeing him and some of the other villains caged at the Avengers Academy, promising him that if he did that, Mephisto and the other villains would serve under him in a new incarnation of the Masters of Evil.
In reality as usual Mephisto was only serving his own interests. After being defeated and caged yet again, Dracula mocked Mephisto for failing once more, Mephisto mocked him back telling him that this was all planned by him and he got what he wanted during the time he was freed.