Leviathans

Leviathans are massive sea creatures generated by The Masked One deep beneath the ocean. They are default hostile, but extremely variable- there is no set "default" leviathan form. From sea serpents to krakens to a whole host of other horrible creations, they slither forth from the deepest trenches of the Great Ocean to test the Aquatic and Surface races alike. Their power levels are wildly variant- some are magic, some are just magically massive; some are clever and have base intellect, others are feral beasts.
Leviathans primarily hunt easy targets such ships and convoys of aquatic races. However, bored Leviathans will eventually seek out larger targets such as Reef-cities or coastal settlements. While they can usually demolish a small coastal or underwater settlement, this typically unleashes the fury of their neighbors. Some extremely patient leviathans anticipate this, and work together in groups to destroy large mortal retribution forces. 
Leviathans are ageless creatures and as such have a long memory. When the first of their kind was slain, the Masked One declared to all the seas that "Only the Strong Survive". This declaration echoes whenever one of their kind falls. Leviathans may not be particularly clever, but those among them capable of understanding kept it close to their hearts. They pass it to one another, as their only divine commandment and law. They are a test to the aged ones, they know- a way to crush the weak. Those leviathans who are too soft, too friendly? They are in violation and must be destroyed, so that another truer leviathan will spawn below.
Leviathan flesh is magically nutritious and filling, and a slain leviathan can feed an entire city. They are hunters and hunted, the cruel logic of the Architects given flesh.