A 42-year-old man presented 6 hours after a thunderclap bifrontal headache with projectile vomiting and confusion. Noncontrast computed tomography (CT) demonstrated diffuse basal cisternal subarachnoid hemorrhage with intraventricular extension, and CT angiography revealed a 7 × 5 mm broad-necked anterior communicating artery aneurysm. After initial stabilization with nimodipine, head elevation, and blood pressure control, digital subtraction angiography confirmed the lesion and an external ventricular drain was placed for acute hydrocephalus. During induction for microsurgical clipping on day 2, profound hemodynamic instability ensued despite vasoactive infusions, and processed electroencephalogram monitoring revealed an unexpected isoelectric trace despite of minimal anesthetic depth. In view of malignant cerebral swelling, electrocerebral silence, and refractory shock, the operative plan was changed to decompressive craniectomy with clot evacuation. This case underscores the importance of real-time multimodal neuromonitoring in guiding intraoperative decision-making for complex aneurysmal subarachnoid hemorrhage.