Erythroblastosis fetalis, also known as hemolytic disease of the newborn, is a potentially life-threatening condition where the maternal immune system attacks fetal red blood cells due to blood group incompatibility, most commonly Rh incompatibility. This condition can lead to severe anemia, jaundice, and even heart failure in the fetus or newborn if not managed properly with interventions like intrauterine transfusions or early delivery.