Immunotherapy is a widely used treatment modality in oncology. Immune checkpoint inhibitors, as a part of immunotherapy, caused a revolution in oncology, especially in melanoma therapy, due to the significant prolongation of patients’ overall survival. These drugs act by activation of inhibited immune responses of T lymphocytes against cancer cells. The mechanism responsible for the therapy’s high efficacy is also involved in immune tolerance of the patient’s own tissues. The administration of ICI therapy to a patient can cause severe immune reactions against non-neoplastic cells. Among them, cardiotoxicity seems most important due to the high mortality rate. In this article, we present the history of a 79 year-old patient diagnosed with melanoma who died due to myocarditis induced by ICI therapy, despite the fast administration of recommended immunosuppressive therapy, as an illustration of possible adverse events of ICI. Additionally, we summarize the mechanism, risk factors, biomarkers, and clinical data from currently published guidelines and studies about ICI-related myocarditis. The fast recognition of this fatal adverse effect of therapy may accelerate the rapid introduction of treatment and improve patients’ outcomes.