The Beta Rift

The Beta Rift is a phenomenon present in the FMP series.

Overview
It is a fracture in the space time continuum that combines elements of the Modern, Post-Modern and Medieval periods. It is also responsible for lands such as Termina being isolated from the rest of the dimension. It is visible in some areas and usually appears to be a giant black tear in the southern horizon. If you get close enough (such as in act 10), it can consume most of the surrounding sky.

Origin
It’s not entirely known why the Rift formed, but it is likely to have been caused by the disconnect of Oblivion and Sovngarde. It has been around for long enough that no one remembers or cares.