Another interpretation of Tartarus of how he would look like.
He's the origin of all the evil ever in Equestria, so powerful the elements of harmony and the power of friendship are useless. He's like King Sombra and Discord on steroids, scary and very competent also a cunning and mischievous super-power of shear evil. His magic would be very Xanatos Gambit riddled, he hates losing and he's a very big jerk so bad Discord's own demeanor is simple filly play.
Watch out other villains big and small because Tartarus won't play nice even to you once freed from his prison.
Extended head-canon: pennywereskunk.deviantart.com/…
Tartarus © PWS13
MLP © Hasbro