The Shadowfell is on a different plane of existance, and is a place of darkness and shadows. The rift was created by The Chosen Five and connects to an unholy sanctuary of Lord Orcus, the demon lord of the undead. Skeletons, ghouls, and even fouler creatures crawled through the opening and wreaked havoc throughout Bael Turath. With the help of Asmodeus, Bael Turath forced The Chosen Five through the rift and sealed it. Bael Turath was destroyed shortly afterwards in their war with Arkhosia.
The Nerath Empire, which emerged after Bael Turath, recognized the danger that the rift represented. Although sealed, the rift could never fully be destroyed. As a result, they built Shadowfell Keep on top of the rift to ensure that it could never be reopened.
The Shadowfell Keep performed without fail until the fall of the Nerath Empire over 100 years ago. When the empire fell, support for the keep was cut off. Within two decades, the keep had been abandoned and fell into decay.
The rift remained dormant until the Death Cult led by Kalarel attempted to reopen the rift using a bloodfilled ritual. Sariah’s sacrifice would’ve completed the ritual and reopened the rift if not for the courage and bravery of The Guardians of the Gate. The Guardians saved Sariah, thwarted Kalarel’s plans, and became mortal enemies of The Chosen Five.