A31 year old male, with a mood disorder, on 30 mg of haloperidol and 100 mg of lithium, is brought to the hospital emergency room with a history of acute onset of fever, excessive sweating, confusion, the rigidity of limbs and decreased communication for a day. Examination reveals tachycardia and labile blood pressure and investigations reveal increased CPK enzyme levels and leucocytosis. He is likely to have developed -