The sad part of this one is that it's the result of a monozygotic twin (or more) pregnancy. There was likely a surviving sibling. Monozygotic twin pregnancy is dangerous, especially if the twins share a placenta and amniotic sac. In this condition, the twins are always monochorionic (one placenta) and sometimes monoamniotic (one amniotic sac).
This happens as the result of a phenomenon called Twin Reversed Arterial Perfusion, or TRAP sequence. The abnormal fetus lacks a heart and is perfused by the normal, "pump" twin, via an arterial anastomosis at the placenta. Reversal of blood flow in the normal twin's umbilical artery brings deoxygenated blood to the acardiac twin. Basically, the normal fetus keeps the abnormal one "alive" but as soon as the umbilical cords are clamped, the acardiac fetus cannot live anymore.
Interestingly, while most surviving twins are normal, 10% or so have a trisomy. TRAP can also harm the normal fetus by overloading the cardiovascular system with too much volume. This results in congestive heart failure and fetal hydrops.
There are other types of acardiac twinning. The acormus type lacks a thorax, and the umbilical cord arises from the rudimentary head. There's an even less differentiated type called acardiac amorphous, which is basically a random assortment of meat.