A sudden thunderclap, "worst headache of my life" with hyperdense blood in the basal cisterns on non-contrast CT is subarachnoid haemorrhage (most often a ruptured berry aneurysm). Hypertensive haemorrhage is intraparenchymal (basal ganglia, pons, cerebellum), and subdural haematoma is a crescent-shaped collection over the hemisphere, not cisternal blood.