Shadow Rifts and Crevices of Dusk

Umberto explained that Vecna used Shadow Rifts unstable tears in reality to travel to the Shadowfell and Beyond.

These Rifts are forced via sacrifice or evil acts but usually temporary.

It is said though that Vecna learned how to create stable ones called Crevices of Dusk from.the Book of Vile Darkness.

The Cult are attempting to create such Rifts or a Crevice by sacrificing Eldon Keyward.