Sonic.EXE also known as X is the main antagonist of the creepypasta Sonic.exe by J.C. The Hyena and its sequel: Sonic.exe/Round 2. It is a demonic entity that possesses a CD-ROM of the original Sonic the Hedgehog, taking on the form of the main character. Then, he proceeds to kill the in-game characters before attacking and removing the soul/killing the person playing his game.
According to the author, this monster is actually a being that spawned in the void, not God or Satan.