The Devil is a scary looking fellow. His face looks sort of like a cross between a human and cow and pig head. His arms are spotted and he has only four claw-like fingers on his hands. He has pointy, reptilian, blade-like growths on his arms. He has horns and pig-like ears. He has small withered up breasts. His abdomen is a face with big, staring eyes. His legs are brown and his feet are clawed.
His left leg claw is dug into someone's back who stands bent over. Just below him are flames, and there are some miserable people in them. There are also a couple of demons who torment the people. A woman stands at the Devil's right foot. She pulls at her hair. A snake looks at her. A man stands by the Devil's left foot and looks up at him. The Devil tosses a scrolled message above his head that says something about redemption. His right hand points up, his left down -- echoing the Magician.
Looking at this card really makes you want to not be in Hell. He controls your Shadow side -- the weak part of you that finds free will too difficult to handle.
We all have to spend some time with the Devil. When he becomes a serious threat is when he prevents you from moving forward with your life, when his claws are dug in deep.