Hollow men are all skin and bones. They have no eyes, only empty holes in their skull to match their mouths, noses, and ears. Their skin has become a hard shell. Wounds become new holes, and do not impede the hollow unless a part is severed.
Where it comes from: Any who dies by starvation and is left unburied or unburned will become hollow.
What it wants and will fight for: Hollow men want meat. They will tirelessly hunt down living creatures, then pull them apart and stuff their meat into themselves. They will ignore living creatures to try to get dead meat, which has led to the common tactic of luring them into groups with animal carcasses and setting them aflame.
What happens if you eat it: Hollow men are tents of dried skin and bone filled with rotting meat. You will get sick.