This humanoid has misshapen limbs—its legs seemingly reversed— and its skull is warped by horns obviously not meant to be there. It leers with a mouth full of fangs.
A Mythos satyr treats its Wisdom or Charisma score as being 2 points higher than its actual value for the purposes of determining the number of bonus divine spells it can cast and the save DCs of its spells if it gains levels in a class capable of casting divine spells, but only if it also worships a Great Old One or Outer God.
A Mythos satyr's mind is as twisted as its body. It gains a +2 bonus on all saving throws against mind-affecting effects, and once per day as a swift action, it can reroll a failed Will save.
When it does so, it must take the result of the second roll, but it gains a +4 bonus on this reroll rather than the typical +2 bonus for being a Mythos satyr.
There are many ways for humanity to become animalistic and vile under the influence of the outside.
Lovecraft's tales show humans degenerating into monstrous quadrupeds in The Rats in the Walls, the cannibal monster Martense family in The Lurking Fear, and Arthur Jermyn. The Milk of Shub-Niggurath notoriously also produces a degenerative effect, and in a very short time.
All of these degenerate genetic mutations have a thousand origins, but all are variants of Mythos satyrs.
Hobbling, chuckling, mewling, and enthusiastic creatures are the frequent result of contact with the outside. A typical Mythos satyr starts out at human size, then grows. They rarely reach colossal size, instead topping out at around 400–600 pounds and some 7–8 feet tall. It is feasible they are limited by their humanoid heritage.
Every Mythos satyr is unique, mixing the human form with some other element. The Mythos satyr's humanity degrades significantly, becoming less and less symmetrical and rational over time. Mythos satyrs may have multiple limbs or limb stubs, and incorporate animal-like or even plant-like forms. The only consistency is their degenerate and mutagenic nature.
The Change When magic or chemicals from outside mix with the blood of humans it provokes changes down to the cellular level. This triggers mutation and eventually causes the victim (who may volunteer for the process) to devolve into a monstrous human entity called a Mythos satyr.
The first effect of the change is typically for any ailments, curses, or ills to be purged (or incorporated into the victim's metabolism). Unfortunately, the victim's mind also steadily spirals into madness as the body undergoes metamorphosis. Indeed, the victim's physical transformation is partly based on his mind, his urges, and his tendencies. Most often, as the physical change takes place, the victim partly comes to resemble some kind of animal, such as a cat, deer, snake, or hawk. Such hybrid creatures may resemble lycanthropes in exterior appearance, but they are internally and mentally very different indeed.
Most Mythos satyrs are the result of a single contact with the outside, and so stop at the earliest stages of mutation, resembling human/beast mixtures. If they continue to contact the outside, the changes to Mythos satyrs do not stop: as they continue to warp physically, their minds degenerate, and they sink into madness.
Over time, the changes become truly grotesque and illogical. Organs begin to sprout on the creature's flesh almost randomly. If the process continues (for example if the outside radiation keeps going for longer periods of time), the victims may just turn into masses of flesh, no longer recognizable as human or even animal.
At this point, the entity's only goals are to consume and reproduce.
Mythos satyrs have animal instincts, but most still have a primarliy human mind. If the changes are allowed to continue, the animalistic part of the satyr becomes stronger as the human part weakens.