This is a blood clot between the brain and the skull, from a ruptured artery, it occurs within 24 hours of head trauma (rarely up to 48 hours). The clot keeps growing, finally compressing the brain, to kill. The trauma usually results in loss of consciousness (maybe very brief), often with a skull fracture, but maybe not. Tony Award winning actor Natasha Richardson died of this; she fell on a beginner ski slope in rural Quebec, hit her head, got up & laughed.
The key to diagnosis is recognizing a “lucid interval” — there’s the trauma, then the patient feels fine, then symptoms get worse & worse. Headache is eventually accompanied by vomiting and confusion.
Anyone with head injury within the last 24-48 hours, whose symptoms are worsening, needs to be brought right to an ER. A simple CT scan makes a rapid diagnosis. Cure is by relatively-simple surgery (since the bleeding is on top of the brain, not deeper in). The key is to do it all in time.