Sundering

A Sundering is a cataclysmic event that causes a super-sized rift to form between a realm and somewhere else; in Sharum Mal'Kari, it is assumed that this "somewhere else" is The Fade.
Nobody truly knows how a Sundering is triggered, but it is assumed to be the result of some sort of eruption (or possibly the opposite-- a starvation) of magical energy at a Nexus point.