So, I saw a bit of discussion about whether eating people should do something nasty to you, and a lot of cultures have a myth about how eating human flesh turns you into a monster. Wendigos, ghuls and gaki spring to mind, but there are others. Cannibalism is a very common theme in mythology, after all.
Here's some mechanics for that. Each time a PC eats a person, they go one step down the Wendigo Sickness tracker. You get the effects of your current level of Wendigo Sickness, and everything from previous levels. Eating the same person more than once doesn't count multiple times. If you were tricked/would have starved if you didn't eat/were compelled, you get to make a Save vs Magic (if you want) and if you pass, you can chose not to go down the Wendigo sickness tracker.
The first meal: No mechanical effects, but the eater knows they will risk becoming a Wendigo if they eat again.
The second meal: No mechanical effects, but roll for Signs of the Wendigo.
The third meal: A mouthful of human flesh is enough to sustain you for a day.
The fourth: You get a bite attack that deals d4 damage, that you can make instead of attacking with a weapon.
The fifth: Each time you eat a person, you heal 1 hp. Only once per person.
The sixth: Roll another Sign of the Wendigo.
The seventh: You get no sustenance from food that isn't meat, & the damage of your bite increases to a d6.
The eighth: You may make your bite attack in addition to attacking with a weapon.
The ninth: Roll another Sign of the Wendigo.
The tenth: When given the opportunity to eat a person (such as somebody you could kill and eat without being found out) but don't want to do so, you must Save vs Magic; if you fail you must attempt to eat them and roll up a new Sign of the Wendigo. No need to save if you're going to eat them anyway.
The eleventh: You heal as much damage as your character level when you eat a person, instead of 1 hp.
The twelfth: Roll another Sign of the Wendigo
The thirteenth: You get no benefit from food that isn't people meat, & the damage of your bite increases to a d8.
Each time thereafter: Every time you roll a Save vs Magic to avoid eating somebody and fail, roll your class's hit-dice; if it comes up as a 1, you transform fully into a Wendigo and become an NPC driven only by their hunger. Roll up a new character.
The Signs of the Wendigo:
Roll d30, take the next down if a duplicate is rolled. Physical mutations emerge over the course of a few days.
- Your feet lengthen, toes warping into hooves.
- Your jaw lengthens into a muzzle.
- You begin sprouting little antlers from your brow.
- Fur grows over your shoulders, upper arms and back.
- The skin peels from your fingers, leaving claws of sharp bone.
- Skin peels from your limbs, revealing muscle and sinew beneath.
- Your eyes become yellow and reflect light like a wolf or cat.
- Your teeth lengthen into fangs.
- Your neck elongates and hunches forwards.
- You grow emaciated and deathly pale.
- You become cold to the touch.
- All your teeth save your canines and incisors drop out.
- You smell of rotting meat.
- Your voice becomes raspy and hoarse.
- Your eyelids and lips recede, exposing your teeth and preventing you from blinking.
- You become immune to the cold.
- Your feet don't touch the ground when you walk.
- All natural animals instinctively fear you, failing all morale checks against you.
- Damage from your bite increases to a d8.
- You are never slowed or hindered by natural terrain such as steep slopes, thick woodland, marshes etc.
- So long as you stand perfectly still, there is a 5-in-6 chance that mundane humans will be unable to perceive you. Animals, wizards, the insane and so on still notice you.
- You no longer need to sleep.
- You become immune to ingested poisons.
- Those damaged by your bite must Save vs Paralysis or be frozen in horror for 2d6 rounds.
- Those who survive your bite are immediately pushed d6 steps down the Wendigo tracker.
- You can see in moonlight or starlight as well as if it was broad daylight.
- You can walk up walls and over ceilings like a spider.
- You can smell people out to a range of thirty feet, and track people by scent like a wolf.
- You never leave tracks, scent or other traces of your passing.
- Your movement speed doubles.