Complex sentence: The men eventually come across a sub-basement, locked from the inside, that harbors something more terrifying and hideous than any of them could have dreamed--a cow-sized queen rat with no eyes or legs, whose only purpose is to endlessly breed more rats.
Split into simpler sentences: The men eventually come across a sub-basement, locked from the inside. As they make their way through the sub-basement they discover that the basement harbors something more terrifying and hideous than any of them could have dreamed--a cow-sized queen rat with no eyes or legs, whose only purpose is to endlessly breed more rats.