Crack

Rifts (た跡) is a term to refer to strange fissures in found throughout the New World and Nazarick in the aftermath of the Catastrophe in Mass for the Dead.

Background
A Rift is a crack formed in the space of reality itself. They are found throughout the New World as a result of a phenomena that coincided in transporting the Great Tomb of Nazarick to its current location. They are found throughout the New World glowing eerily purple, emitting strange energy.

Mechanics
So far the cause of this phenomena is unknown, but what is known by the inhabitants of the New World is that monsters that go near the fissure go wild.

According to one theory, there seems to be some kind of poison that is leaking out from the crack, affecting organic matter creating a new breed of mutated creates known as Contaminated Beasts. Infected creatures are not only the danger, as these rifts give birth to unique creatures known as Chaos Beasts. These being are wrought entirely of a strange substance known as Chaos Stones. The rifts act as gateways to the New World to another dimension known as the Chaos Zone. Occasionally the Chaos Zone spills out into the New World, for brief periods, bringing legions of its own ilk, before retracting back into itself.

Additionally it has been noted that the Rifts have disrupted the magical laws of the New World. In fact transfer magic is difficult and may not work. Resurrection magic is also impossible as discovered by Nazarick.

Trivia

 * A Rift has permanently formed in the 10th Floor of Nazarick, right where the Throne of Kings once sat.