Intracranial hemorrhage
A subarachnoid hemorrhage is bleeding into the subarachnoid space (the space between the arachnoid and the pia mater).  ("Subarachnoid" means "under the arachnoid.")  Because the subarachnoid space holds cerebrospinal fluid, bleeding here makes blood mix into the cerebrospinal fluid.  The blood irritates the brain and spinal cord, and causes symptoms like a very bad headache and a stiff neck.