Mephisto is a creature of indeterminate origin that rules over an abyssal dimension called Abaddos. It is a terrible and malevolent location, with a harmony that makes it adjacent to the dimension of Hell.
Mephisto styles itself as an incarnation of Satan, taking on a male aspect and bargaining power to mortals in exchange for their souls. He has amassed a tremendous number of souls in his dimension, giving him incredible power beyond even his authority over his home realm. It his ambition to amass enough power and influence that Abaddos becomes akin to Hell. Given enough time and souls and suffering, it's possible that his realm might slide into Hell and his dominion over that demesne might allow him to challenge the Lords of Hell themselves.