'coldest realization' is a realization that is a mixture of shock and perhaps dread/fear knowing something bad is about to happen.
When you are in shock or a sudden fear, there is a body response of feeling cold because blood flow shifts from your extremities/face to your body core. This is a natural response to fear or shock. You can feel cold and even shake.
The 'coldest realization' draws a comparison to this. The comparison may be metaphorical or literal. In either case, it is saying the shock is sudden and deep.