Deep Ones are one of the more famous and iconic monsters to come out of the Cthulhu Mythos. First appearing the Lovecraft story "The Shadow over Innsmouth", the Deep Ones are a race of amphibious humanoids - described as looking like a humanoid amalgamation of fish and frog - who inhabit the depths of the oceans, where they worship Cthulhu and other Great Old Ones. Ironically, Deep Ones are amongst the more human denizens of the hidden darkness; they pursue a lifestyle not that different to humans, seeking comfort, wealth and the chance to do what their cults consider "good deeds". As such, they are capable of conducting diplomacy with humans and forming alliances; the precedent set by Innsmouth amounts to the Deep Ones offering good harvests of fish and bounties of treasure retrieved from the ocean's bottom in exchange for conversion to their faith and other acts of allegiance.
One of the more unusual aspects of Deep Ones is that they can interbreed with humans. These Deep One Hybrids spend the first fifty years of their life as humans, albeit with a pale and/or greenish tinge to to the skin and somewhat batrachian features (the so-called "Innsmouth Look"); most fictional portrayals keep both of these qualities in some shape or form. As they approach their 50th birthday, the Hybrid's features become more and more inhuman, until they reach the age of fifty and transform into a full-fledged Deep One in turn. As Deep Ones possess immensely long, if not infinite, lifespans, this is seen as a great blessing; once they have transformed, the former hybrid is functionally immortal.
So why are Deep Ones seen as the most evil of the Mythosian monsters? Let's just say that Lovecraft had some... "peculiarities" (as in his sole criticism of Adolf Hitler was how optimistic the guy was), and people still tend to buy the unflattering light he portrayed them in.
Dungeons & Dragons expied the Deep Ones as the Kuo-toa (and, to an extent, the Skum)... oh, and the 5e version of the Sea Spawn, too. Pathfinder just brought them in wholesale, and even made Deep One Hybrid a potential PC race. That said, this is basically the even grimdarker aquatic version of the Half-Orc, since you will ultimately transform into a full-blown Deep One, and this means complete, total and irreversible personality death. And also 'actual' death, sort of.
Monster Hunter International Deep Ones are far more monstrous. They raid ships on the surface and shoreline to forcibly implant their eggs in human hosts that burst open Aliens style to spawn horrifying monsters. They are so widely despised every nation on Earth with a submarine has agreed to cooperate in dropping depth charges on their cities when they are discovered.
Pathfinder Deep One Hybrid Stats
They are most comfortable with others of their kind, and typically cluster in small, insular settlements where they can assume positions of authority. They keep any humans within their towns subservient and cowed, making sure those other residents know better than to act against local laws. Though deep ones are devoutly religious, they usually cloak their true beliefs under a facade of more conventional worship, believing that by doing so they can avoid suspicion from visitors and nearby societies.
- Ability Score Modifiers: Deep one hybrids are hearty and devout, but tend to be somewhat slow and awkward in their movements. They gain +2 Constitution and +2 Wisdom but suffer –2 Dexterity.
- Type: Deep One Hybrids are Humanoids with the Human and Deep One subtypes.
- Size: Deep One Hybrids are Medium creatures and thus receive no bonuses or penalties due to their size.
- Speed: A deep one hybrid’s base speed is 20 feet, as it walks with a shuffling gait. A deep one hybrid has a swim speed of 30 feet, and gains a +8 racial bonus on all Swim checks.
- Languages: Deep one hybrids begin play speaking Aklo and Common. Hybrids with high Intelligence scores can choose from the following languages: Abyssal, Aquan, Dwarven, Elven, Giant, Gnome, Goblin, Halfling, Orc, and Undercommon. See the Linguistics skill page for more information about these languages.
- Natural Armor: A deep one hybrid’s thick skin and dense fatty tissues grant it a +1 natural armor bonus to AC.
- Final Change (Su) A deep one hybrid ages at the same rate as a half-orc. A mere 1d12 months after a deep one hybrid reaches venerable age (at 60 years old), it dies a painful, agonizing death, only to have its body transform into that of a mature deep one. This transformation functions as the reincarnate spell, with the newly formed deep one gaining the following modifications to its physical ability scores: +6 Strength, –2 Dexterity, and +6 Constitution.
- Sea Longing (Ex) Every 24 hours a deep one hybrid spends in an area more than 10 miles from the sea, it must succeed at a DC 20 Will save or take 1 point of Wisdom drain.
- Take to the Water (Ex) A deep one hybrid can hold its breath 10 times longer than a human can, and gains a +2 bonus on Initiative checks and Reflex saving throws while swimming.
- Low-Light Vision: A deep one hybrid can see twice as far as a human in starlight, moonlight, torchlight, and similar conditions of dim light.