Welcome to PF;
Damage to the human body is generally a complicated subject so you have picked a great topic for an essay.
Is this a physics/engineering or a biology/medical essay?
Basically your head is a thin shock-absorber over a hard shell with a bunch of delicate tissue in liquid inside it. So there are several things that can be damaged. The biggest thing to worry about is bruising through the brain - eg. if the force of the blow throws the head back hard, the law of inertia has the brain hit the front and then the back of the skull-cavity. Once bruised, the brain can swell up. Since the skull of an adult won't flex, this swelling can be fatal.
You also need to worry about shock-waves through the head and, of course, possible fractures.