Blasphemous Fish-Frogs of the Nameless Design

Reading the description of the drider teeth from Emmy Allen's game, I got into the idea of using these side progression paths as a source of monsters. The first one that I finished was an old pulp classic from the Shadow Over Innsmouth - Deep Ones.

Technically, compared to the Lovecraft source material this turns you into a Deep One hybrid and not a Deep One, who are the blasphemous fish-frogs of my title, but this was the much cooler quote and the hybrids are the only ones I care about in my setting.

The Progression

There are six powers of the Deep One that you gain in a random order as the secret heritage takes hold. Roll a d6:

1. You have a phenomenal power of smell, especially for blood, and may track it over long distances. In addition to narrative effects, take +2 on Tracking rolls involving smell.
2. You have phenomenal vision - not reaching much farther than normal humans, but resolving incredibly fine detail and seeing in darkness and through murk. Take +2 on Perception.
3. You can jump mighty distances easily and reflexively. Any part of your movement during combat may include vaulting minor gaps and obstacles without a roll or wasted time. Jumping rules are always weird as shit, so scale to match whatever system you’re using, but for my purposes we’ll say you can easily jump six feet vertically, fifteen feet from a standing long jump, and a running jump can net you thirty feet. If you need to make an Athletics check for some ambitious jump, take a +2.
4. You can communicate with fish and amphibians as though you had Speak with Animals.
5. You can move at full speed through water unless heavily encumbered, which no longer brings inherent risk of drowning.
6. You face no risk of fatigue in wet weather, and are always considered to be wearing warm clothing in the face of cold weather, even if you’re naked and dancing about in frantic worship.

After the first power, their eyes bulge out and grow unblinking, and they begin to go bald.

After the second power, they grow some gills. At this point they’re sinking into their new life and other kinds of advancement come harder for them. Double the XP cost of gaining levels. Most if not all hair is gone, their ears are flattening, and their head is narrowing.

After the third power, they gain a +2 on wrestling rolls (or +3 if they’re underneath or standing in water). All body hair is gone at this point.

After the fourth power, the dreams start. Every night the Deep One sleeps, roll a d6. On a 1, they get a vision. Treat this as a single question per Contact Outer Spheres, with a 6+ on d20 chance of being a truthful omen and a 15+ chance of contracting madness from the table below. The dreams always involve all the tone and aesthetic you’d expect. Their head is entirely changed and their jawline is inhuman.

After the fifth power, they grow scales all over their body, granting them +3 AC. Any slim remaining chance of being mistaken for human is basically gone at this point.

After the sixth power, they gain an extra hit die of flesh. They gain no benefits from level gain afterwards except for more hit dice (which go to flesh) and improved saves.

Roll a d10.

1. You grow depressed any time you're far from the sea, lake, or at least a river for more than a few days.
2. You consider insects (and especially flies) to be incredible delicacies.
3. You have to at least get your face damp in order to get a good night's rest.
4. You see visions of your people's past when you sleep, filled with vicious hunting parties and cyclopean architecture. You long for such a time to return.
5. You believe some progenitor or elder god will catch and judge your soul when you die.
6. You believe that you are unnaturally vulnerable to some harmless object, and that it can kill you with merely a touch.
7. You struggle to remember that others do not like your changing looks, since you and your fellows are so self-evidently beautiful and glorious.
8. You sometimes have nightmares of the sun growing heavy and huge in the sky, red rays burning away the seas and condemning you to a miserable death.
9. You grow sexually impotent unless you're at least standing in water.
10. You feel compelled to perform rituals honoring your progenitors at least once a week, which grow increasingly involved and strange over time.

Fitting Them In

These little charmers can slot into a campaign world in a bunch of different ways. In mine, they're not the product of eugenic sin but the servitor/warrior -caste of a old sorcerous priest-king whose lost city lies sunken off the coast, their heritage and secret plans hiding out among the local bands who cast him down generations ago. 

How you become a Deep One should vary based on what you need them to do, from logistics of their plans to what kind of fears they need to represent. If you've got the kind of players who don't mind trading looks and a bit of sanity for practical powers, they make a solid option for them, and they might be spread like a werewolf bite or an occult ritual rather than being inherited.

Whatever the process and trigger, every time they stand a chance of progressing, make a save against magic; if they succeed, they don't mutate further. 

'Classical' examples translate somewhat messily into a stone age setting because 'middle age' is a very different concept for them; the best adaptation IMO is to make it a condition associated with the elders of certain tribes.