It's a Byzantine coin. Not Kidding : Byzantium, 3rd c. BC Obv.: Head of (youthful) Dionysus, right, wearing ivy wreath. Rev.: ΒΥΖΑΝΤΙΩΝ / ΕΠΙ / ΑΣΩΠΙΟΥ, nude Poseidon standing facing, head left, chlamys over left shoulder, holding Nike standing left in outstretched right hand and trident in left arm. Large circular countermark obliterating the right half of the reverse. https://ikmk.smb.museum/object?lang=en&id=18234904