Mastoiditis. If left untreated, true suppurative acute mastoiditis, could lead to extracranial (facial paralysis, labyrinthitis, petrositis, subperiosteal abscess) and/or intracranial (meningitis, brain abscess, extradural abscess, subdural abscess) complications.
Potential badness. Potentially serious complications, including meningitis, brain abscess, permanent hearing loss, death. Caveat: "mastoiditis" diagnoses radiographically only (ct or mri) is not necessarily the mastoiditis I'm talking about. If you have true clinical or coalescing mastoiditis, you would be in hospital right now. If it's from a ct report, you should talk to your pmd or see an ENT to clarify.