The House on the Borderland - Weird Fiction by William Hope Hodgson

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 it's even stranger, jade-green double, seen by that old man on an otherworldly plain where gigantic gods and monsters roam. Soon his earthly abode is no less terrible than this strange vision, as swine-like creatures boil from a cavern beneath the ground and besiege it. But a still greater horror will face the recluse, one more awful than any creature that can be fought or killed.