From publisher blurb:
The Crawling Sea, Ryug-Tnug the Black Ocean, spreads across the seafloor under the Nyxian Ocean about ten miles west of the southernmost tip of the Endless Steppe. An ancient, sapient ooze of gigantic, impossible size, when it stirs from its slumber, destruction follows. The last time the Crawling Sea awoke, it emptied the Endless Steppe of the people living there, leaving it open for the centaurs to take. This time, the centaurs stand imperiled, and so too does the rest of the world.
Some foes occupy a class of their own. So powerful are they that not even dragons, giants, or other great monsters can threaten them. The Crawling Sea introduces an entirely new and awful threat to Shadow of the Demon Lord, of such immense potency that it can draw the Shadow onto itself and blight the world. With a new enemy, new underlings, and dread secrets revealed, The Crawling Sea can bring deeper darkness to your games.