We Plundered Satan's Closet And Found This...

The Adepts have, for whatever reason, come into possession of a Dark Object.  Maybe it's a spellbook, a Grimoire written by some great evil beyond the stars or the blackened heart of Judas Iscariot.  Whatever.  The point is that it is a device powered by Evil and that the Adepts can't destroy it by any means currently available to them.
It's presence is causing supernatural problems and it keeps attracting agents of the Darkness to the players.  Think of the one ring in LOTR.