The Deep Kin are a species of semi-amphibious fishmen from the deep ocean. They are very intelligent and aggressive creatures, known for being highly territorial and wary of outsiders. Given the large ecosystem of the ocean waters, the Deep Kin have many predators, but do not mistake them for prey. They are a carnivorous hunting species that are sufficient tool users and can fend off larger predators. But most often perish. Many consider them to be cold, callous, and cruel, but have little reason to trust or even be friendly to others.

The Deep Kin also have a religious cult that seems to be shared across many of their clans, no matter how far apart. But there are many clans considered heretics for rejecting what are essentially their creators and birth parents. The Deep Kin worship two beings. One known as Sidon, The Deep One, or the Father of the Deep. An ancient eldritch entity said to sleep beneath the crust of the ocean floor, who whispers to them in their sleep. His mate is Meltist, the Mother of Many, or Mate of Sidon.

Most Deep Kin live in a patriarchal society, and their numbers are comprised entirely of males. Female Deep Kin exist, but they are a rarity and are fiercely guarded over by their male kin. Many females are confined to their roles as shamans, caretakers, breeders, and queens. Queens can lay up to 1,000 eggs a year. Out of the 1,000 eggs, about 1/4th are female. There is a 65% mortality rate among the Deep Kin's offspring, with many contributing to natural predators, rival clans, and parasites, while others are marked as sacrifices to their gods in exchange for power and or magical protection.

Despite the massive difference in the gender birth ratio and the high mortality rates, most Deep Kin are mostly monogamous and are encouraged to seek out female mates of other species in both sea and on land through seduction, abduction, or even making deals with the local communities.

After a Deep Kin acquires a mate, they are said to mate for life and will choose no other female to mate or reproduce with. They will be fiercely protective and ensure the needs of their mates are well met. Although at times, during a period of their mate's pregnancy, Deep Kin will often seek out lesser males for pleasure, and as servants of their homes while the patriarch goes out to hunt for food.

Deep Kin heretics, especially ones that integrate within different communities on land, are said to be less violent and aggressive.