Lullaby

From the publisher's website:

In the fourth and final Act of Big Sleep, Lullaby, Tsathoggua’s rampage continues unchecked. Your heroes must delve deep into the caverns whence the Great Old One emerged. Here, you encounter ancient subterranean civilizations and primordial beasts. As you make your way to the lightless realm of N’Kai, Tsathoggua’s origin, you find terror, abomination, and stygian vaults. But somewhere in this nightmare realm is the secret behind the Sleeper’s power. Only you and your heroes have the skills to obtain it, and the knowledge to use it. But Tsathoggua is also aware and alert – will you be ready for the final showdown with the titan itself and its army of slithering monsters?