The main idea is described here
However, to summarize, the army of a a prince from one of the Human refugee kingdoms in the Dreamlands comes back to earth (basically as human disney-red-martians) to assist the Deep Ones with a war against a frightening new enemy, the so called "Dark Conquerors" (some hints from Acheronian sorcery, some from Marvel-movie-verse Chitauri).
Now, the Deep Ones rule and Cthulhu is awake, so multiple avenues can be taken in such a setting. For Horror, one could say that unlike their prince, Korpu, his men weren't raised by a nanny who was a MASK, and thus are fully vulnerable to Cthulhu's psychic echos. The longer they stay to fight, the worse they get and the more turn their weapons on themselves rather than damage their cause. But one could also run it as a planetary-romance-without-the-romance, with great battles against horrible foes and mysterious origins. Even classic pulp could lend a hand as spies try to find the origins of this invasion and weaknesses of their foes.
Or you could scrap all that and just use the setting the way you see fit.