The Nature of Rifts (Rift Lore)  

Rifts appear where the boundary between the material world and the elemental plane has grown thin and tattered. They manifest as the forced opening of a tear in the Ward. It is observed that from the moment the Ward broke, the frequency and intensity of rifts grew exponentially, as the existence of rifts created more rifts.

Rifts allow members of the planes to spill into Telara, but even more importantly, they draw the plane itself closer to Telara. This is what is known as convergence; Telara is flooded with too much elemental energy, shattering the homoeostasis of elemental energy responsible for the world's bounty.

When Regulos destroyed the ward completely he created an almost convergence of Telara with the Plane of Death. Only a very small sliver of Telara remains.