The Creeper Beneath

From the introduction:

Water filled tunnels. Sludge at bottom makes it hard to move fast and quiet. The tunnels are the lair of an unholy abomination, long ago worshipped as a god by now, mostly dead cult. "The Creeper Beneath" is terrifying, invisible giant spiderlike thing. You'll hear it splashing as it approaches, its mandibles clicking like bony but wet fists against rotten flesh. It likes to play w. its prey...