Satan is the final boss of the game and found at the lowest level of Mars.
When you find (and fall into) the hole at the bottom of Mars, Satan reveals he is the one who has been killing miners that dared venture into the bottom. After his true identity is revealed, the fight begins.
Satan has 1000HP during his first phase, and turns into a more demonic form with 2000HP once the first phase is beaten. Once defeated, you gain a very huge money bonus, and can start over again. (You can fight Satan 99 more times, but each time you fight, his HP is multiplied by the number of times you have defeated him +1, so the second time his health is 2000/4000, the third his health is 3000/6000). Plus, the damage value is determined by the levels as well.