Mephiles the Dark is the main antagonist of the 2006 Sonic the Hedgehog video game. He was accidentally created during the Solaris Project. Mephiles took Shadow's form after being resurrected and never used anybody else's appearance. Mephiles is the mind and will of Solaris while Iblis is the raw power. They eventually fuse back together into Solaris and are later defeated. According to Ian, the Time Eater, one of the antagonists of Sonic Generations, is actually what's left of Mephiles, who managed to slip through a crack in time. While this applies for the comics, it's unknown if it's true for the games, but Ian states that it is impiled for the games, considiring the hints and similiarities of the Time Eater's appearance and its abilites.
Mephiles the Dark is the only villain who was able to kill Sonic the Hedgehog, even though, Sonic got brought back to life again.