Deep One Culture
Sandy Petersen's Cthulhu Mythos - Pathfinder
The deep ones are an incredibly strange species. Though they live undersea, they are capable of operating on land. They are unusually fecund and can interbreed with other species, producing (eventually) more deep ones rather than hybrids. They are able to grow (or shrink) to different sizes, based on diet. They have their own culture, ancient and independent, yet integrally tied to and invested in humanity and other mortal cultures.

The key to the deep ones' strange nature is that they are an artificial species, created by Great Cthulhu to interact with and possibly supplant humanoids. Although not an ancient race compared to many in the Mythos, their culture is older than those of most humanoids. Physically, they are roughly similar to humanoids (in that they are bilaterally symmetrical and have four limbs), so that they can interact more effectively. They use vocal sounds for speech.

Deep One Life Cycle

The deep ones are able to reproduce with almost any other species of appropriate size. Initially, the resulting young look like the non-deep one parent. Thus, if a deep one mates with a shark, the spawn looks like a shark. If it mates with an elf, the spawn looks like a baby elf.

Over time, as the spawn grows and matures, eventually it will begin to make the transformation into a deep one. In humanoid races, such as orcs, humans, or dwarfs, this starts usually a decade or two after the person reaches physical maturity and takes a significant amount of time to complete. In humans, the hybrids would usually be in their 30s before the metamorphosis begins, and in another decade they resemble deep ones. Psychic or emotional trauma or shock can accelerate or stimulate the change.

The more different the mate is physically from the deep one (particularly in terms of bone structure), the longer the metamorphosis takes. For example, a dolphin may take two or three times as long to complete the change as a human.

Once the deep one change is complete, the entity typically joins the rest of the deep ones beneath the waves. Ideally, the deep ones are aware of their spawn and keep in contact with them, even putting together social constructs to educate and control such individuals (such as the Esoteric Order of Dagon).

Malformation and Error

The transformation process does not always go well. Some hybrids never manage to complete the change and stall partly into the process. Other genetic errors can occur, and sometimes disease or heredity affects the change for the worse. These malformed hybrids rarely gain a formal position within the deep one society but instead tend to be hangers-on. The deep ones or more fortunate hybrids sometimes use them as tools or pawns. Under the Sea

Deep ones are generally one of the most socially and magically sophisticated races of the sea, in their own unsettling way. They typically construct large and complex cities deep under the waves. When these are located near a shore, the local population is often subverted by the deep ones in many ways.

Deep one society is stratified and controlled by its older, higher ranking members. One feature of deep one biology is that, if fed sufficiently, they continue to grow to indefinite size. Their leaders ensure that this only proceeds under their control: only those deep ones that are needed to be huge in size are permitted to grow unhindered. On occasion, there have been entire cities of gigantic deep ones, but in a typical city, only a few individuals exceed the normal size because a large population of huge deep ones requires more food than most cities can support. Also, giant deep ones cannot breed easily with the mortal races.

When deep ones grow to Large size, they do not necessarily stay humanoid in appearance. Their body form can continue to mutate and adapt, and so extremely large individuals may look very different from their smaller kin (for instance, Dagon and Hydra). The deep ones have great skill in biology and genetics, and thus can often control and direct such growth to suit their nefarious purposes.

Deep ones are also capable of domesticating and using other types of undersea creatures, particularly Mythos beings such as shoggoths.


The deep ones maintain constant surveillance and contact with the surface world near their homes. Their ultimate purpose is not their prerogative, but determined by Cthulhu’s design. They not only monitor mortal activity for the Great Old One, but may someday be used to supplant the mortal realm.

Their hybridization and inbreeding is one important tool toward that goal, for it allows them to blend in with mortals. The hybrids, of course, knowing they will one day be full deep ones, are fully supportive of the deep ones’ goals.

Deep ones are known to have originated many magic items, including the medallion of Y'ha Nthlei, seal of Hydra, starstone of Mnar, and tiara of Mnar. They also practice a profoundly dangerous method of creating twshas to control shoggoths and proto-shoggoths.

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