The House on the Borderland

A manuscript is found: filled with small, precise writing and smelling of pit-water, it tells the story of an old recluse and his strange home - and its even stranger, jade-green double, seen by the recluse on an otherworldly plain where gigantic gods and monsters roam. Soon his more earthly home is no less terrible than his bizarre vision, as swine-like creatures boil from a cavern beneath the ground and besiege it.
But a still greater horror will face the recluse - more inexorable, merciless and awful than any creature that can be fought or killed.