Atlanto-occipital dislocation
Atlanto-occipital dislocation, orthopedic decapitation, or internal decapitation describes ligamentous separation of the spinal column from the skull base. It is possible for a human to survive such an injury; however, 70% of cases result in immediate death.
Source: Wikipedia — Atlanto-occipital dislocation (CC BY-SA 4.0)