Malignant pheochromocytoma/paraganglioma is a rare type of tumor that develops in the adrenal gland or in the sympathetic nerve tissue outside the adrenal gland. It is characterized by the overproduction of hormones like adrenaline and noradrenaline, which can cause high blood pressure, palpitations, and other symptoms. If left untreated, the tumor can spread to other parts of the body and become life-threatening.