Why is an epidural hematoma so deadly?

Risk of Herniation: Epidural hematoma entails blood building up outside of the brain. Because brain is confined within rigid skull, accumulation of blood will gradually cause an increase in pressure within the skull cavity & compress brain tissue; it can even cause the brain to herniate out through the skull where the spinal cord originates, leading to a coma and/or irreversible brain damage.

Large blood clot: Epidural hematomas are deadly because they can accumulate very rapidly. These frequently arise from a tear in an artery along the lining of the brain-the dura. As this is arterial, blood comes out very rapidly. It can reach a point where there is significant compression on the underlying brain. This leads to brain dysfunction. This frequently occurs over the temporal region.