Key Inclusion Criteria:  18 years of age or older at the time of signing the informed consent. Body weight ≥ 30 kilograms at the time of Screening. Male or female. Diagnosis: Meet American College of Rheumatology/European League Against Rheumatism classification criteria for definite or probable DM. Participants who have an inadequate response (that is, continued impairment by medical doctor report) or are intolerant to 2 or more DM treatments, including systemic corticosteroids or immunosuppressive/immunomodulatory therapies (for example, azathioprine, methotrexate, rituximab, intravenous immunoglobulin), either in combination or as monotherapy. Vaccinated against Neisseria meningitidis within 3 years prior to initiating ravulizumab as per national and local guidelines. Participants must receive the vaccination at least 2 weeks before first study intervention. The sponsor recommends that national and local guidelines for prophylactic antibiotics should also be followed. Female participants of childbearing potential and male participants must follow specified contraception guidance as described in the protocol.  Key Exclusion Criteria:  Cancer-associated myositis, defined as the diagnosis of myositis within 3 years of the diagnosis of cancer (except basal or squamous cell skin cancer or carcinoma in situ of the cervix that has been excised and cured and at least 3 months before Screening). Evidence of active malignant disease or malignancies diagnosed within the previous 3 years including hematological malignancies and solid tumors. Participants with other forms of myositis. Participants with significant muscle damage (for example, severe muscle atrophy, end stage muscle disease) as per investigator opinion. History of Neisseria meningitidis infection. Human immunodeficiency virus (HIV) infection (evidenced by HIV Type 1 or Type 2 antibody titer). Active systemic bacterial, viral, or fungal infection within 14 days prior to ravulizumab administration. Presence of fever ≥ 38°Celsius (100.4°Fahrenheit) within 7 days prior to study drug administration on Day 1. History of hypersensitivity to murine proteins or to 1 of the excipients of ravulizumab. Pregnant, breastfeeding, or intending to conceive during the course of the study. Inability or unwillingness to adhere to the protocol requirements.