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Background Eosinophilic myocarditis (EM) is a rare and devastating condition. The underlying cause of EM is unknown, and the natural history is not well understood. Case summary A 20-year-old male presented in cardiogenic shock with preceding 24-h history of pleuritic chest pain associated with nausea and vomiting. Electrocardiogram showed sinus tachycardia with widespread ST elevation, significantly raised high-sensitivity troponin T, and raised white cell count with eosinophilia. Transthoracic echocardiogram demonstrated severe left ventricular (LV) impairment and a moderate-sized pericardial effusion. Right ventricular (RV) endomyocardial biopsy and bone marrow biopsy were performed, with both demonstrating prominent eosinophilia. He was initiated on pulse methylprednisolone leading to rapid clinical improvement with normalization of LV function. Day 9 after discharge, he was readmitted to hospital with presyncope and right heart failure. Electrocardiogram revealed junctional escape rhythm, and cardiac magnetic resonance imaging showed scarring confined to the atria. The patient was treated with mepolizumab and underwent an electrophysiology study with electroanatomical mapping, demonstrating sinus arrest and the absence of electrical activity throughout the right atrium. After much deliberation, an implantable cardioverter-defibrillator was implanted with a deep septal RV pacing lead and an apical RV defibrillator lead. Discussion We present a unique case of EM with two distinct phases: the first marked by severe LV impairment resolving with immunosuppression; the second characterized by atrial cardiomyopathy leading to persistent symptomatic sinus arrest necessitating permanent pacing. Close follow-up of EM after initial remission is essential to monitor for further complications including heart failure and arrhythmias. [ABSTRACT FROM AUTHOR]