Forebears' Holdout is a massive cave in central north-western Skyrim, across a river from Dragon Bridge. Inside, there is an occult altar and small fortress built into the farthest interior wall. The interior of the cave has a running stream and a variety of plants and minor fauna.
Due to the presence of the fort and altar, the cave is occasionally used by certain parties with dark intent, such as renegade vampires.
The Weystone Source can be found here.
Note: The cave only becomes accessible during Prophet.