Acute necrotizing encephalitis (ANE) is a rare and life-threatening form of encephalitis associated with influenza virus and other pathogens. It is characterized by a rapid onset of neurological symptoms and has been linked to a cytokine storm within the brain. We present a unique case of an eight-year-old female with influenza B-associated ANE, involving multiple brain areas including the cerebellum and brainstem and cauda equina involvement. The patient had a rapid neurological deterioration, and MRI findings revealed extensive multifocal areas of abnormal brain parenchyma and inflammation with Guillain-Barre appearance in the cauda equina. To the best of our knowledge, this is the first reported case of ANE with cauda equina involvement leading to neurological deficits. Despite treatment with oseltamivir, steroids, and intravenous immunoglobulins, the patient had poor neurological outcomes, similar to those reported in the literature.