Leviathan: The Tempest
In short, Lovecraftian Kaiju.
In the beginning, the world consisted of formless chaos, the Primordial Waters. At the heart of the sea was Tiamat, the mother of all things. Tiamat and her children, the Progenitors, ruled the Primordial Waters and were worshiped as gods by mankind. Then came Marduk to do battle with Tiamat and the Progenitors, in a climatic conflict he slew them and reforged the world drawing land and sky out from the Primordial Waters. Or so we all heard.
While playing as sea monsters and demigods would be fantastic on paper, the setting shows some Catch-22 instances. Leviathan is a game about living in a world the hapless PCs are entirely unsuited, with dual themes of puberty and old age. To be more specific, two problems for Leviathans exist. Freshly transformed and whelp Leviathans must adapt to physical changes, new powers, and responsibilities after they undergo their unlucky transformation. If you think the elders have it easy, think again! They must cope with the fact that as a people, they've been going the way of the dodo, yet accomplished nothing of value.
Rubbing further salt into their wounds, the Cthulhu wannabes have been split into different splats after Marduk mopped the floor with their grandmas and grandpas. Each splat represents one of the seven deadly sins and an associated vestige of what made them mighty eons ago. Their bugbears for the big fish are hunters from the Marduk Society, remnants from the disciples of Tiamat's slayer.
Starting off as a joke proposal for a Creature of the Black Lagoon gameline, the fan project was taken seriously and snowballed into the Lovecraftian splash-fest players see today.