A 31 - year - old male, with mood disorder, on 30 mg of haloperidol and 100 mg of lithium, is brought to the hospital emergency room with history of acute onset of fever, excessive sweating, confusion, rigidity of limbs and decreased communication for a day. Examination reveals tachycardia and labile blood pressure and investigations reveal increased CPK enzyme levels and leukocytosis. He is likely to have developed: