Satan, which means "the Accuser", is the primary antagonist of the faithful in Abrahamic religions.
In the Hebrew Tanakh, what Christians refer to as the Old Testament, Satan's role is that of a judge of mankind. He is not a fallen angel (though verses in Ezekiel hint at this) but one of the heavenly host who, true to his name, acts as an accuser, akin to a prosecutor in a courtroom today. In apocryphal Jewish texts such as the book of Enoch, Satan did indeed fall from heaven for being jealous of God's creation of Adam and Eve.
In Christianity, he rules over Hell. Overcoming his temptation is the goal of believers and his defeat will signal the coming of paradise. In Islam, he is referred to as Iblis or Shaitan.
In Video Games
Satan made an appropriately metal appearance as " Lou", for the final boss battle in Guitar Hero III: Legends of Rock. He is also featured in the "storyline" of the career mode for Guitar Hero III. He first appears as the agent that signs the player's band to a label, and also frees their band from prison, taking them on a whirlwind tour. Eventually, when the player is famous enough, their character tells Lou that they don't need him anymore. It's at this point that he turns out to be Satan himself, and challenges the player to a guitar duel for their soul!
The Binding of Isaac
In the PC game The Binding of Isaac Satan appears randomly offering items to Isaac, including "The Pact", a presumable deal with the devil that turns his skin black, but grants him greater abilities in combat. Satan also appears as a two-part boss monster in the content added in the Halloween update to the game. If the player attempts to use the Bible on him they will be killed instantly.
Sam & Max
In the Sam & Max games Satan appears as a very comical figure. Instead of being a mythological environment of fire and brimstone hell appears as an average company office and instead of appearing as an evil overlord, Satan appears as a rather average office manager.
Like many figures of religion and mythology, Satan makes recurring appearances in the Megami Tensei franchise. He first appears as the penultimate boss of Megami Tensei II, where he stands before only YHVH himself. In Shin Megami Tensei II, he further acts under YHVH's command as the necessary tempter to keep mankind under control and as a judge to weigh their actions.
Satan also appears as an optional boss in Digital Devil Saga 2, and is a high level persona in the Persona franchise. In the Persona 2 titles Innocent Sin and Eternal Punishment, as well as Persona 3, he is half of the pair needed to cast Armageddon, an instant-kill move. He also appears in Persona 4, though the game does not feature combination abilities in the vein of previous games.