The Devil Cursor is a cheat boss. You can summon him by getting 666 cursors and then typing game.spawnDevilcursor.devil. He will possess your cursors and can make devil cursors. The only way to beat him is clicking twice and then hitting every key as fast as you can on your keyboard (a la Candy Box!) One key will take HP off him. Keep pressing that button rapidly until he goes away. The reward for defeating him is 987654321 cookies. (Or if you're fighting his harder version you get 12345678900987654321 cookies!)