Taking on the strength of an animal is a pretty common part of mythology, whether by wearing its skin, eating its heart, or performing some other strange ritual with its remains.
In my current campaign, I'm toying with the idea of introducing the ritual consuming of both monster and man in order to gain their powers - a blasphemous act, but one that can give you great strength. As a pre-warning, this is NOT playtested and while I'll post an update with how it goes, right now it's essentially unbalanced. I'm fairly sure it will also place a bit more of an emphasis on combat as it increases the rewards. Hopefully the drawbacks will make it less of a no-brainer and more of a tradeoff. At the same time, I want this to be used so the drawbacks can't be too intense.
When a character slays a monster, they may loot the corpse for a symbolically valuable body part relevant to the creature - the heart of a lion, the skull of an animated skeleton, a thimbleful of ooze, the eye of a beholder.
They can then perform a ritual to extract the creature's soul, transform it into an edible substance and then devour it.
The ritual (specific to each creature) takes about ten minutes to perform, and then the player rolls on the below table.
To boost the ritual, a player may;
+1 - use a superior, rare or powerful specimen of the creature.
+1 - detail an appropriate and fitting ritual that they perform as they consume the part.
+1 - sacrifice something - blood, valuables or something more esoteric.
Consumption of Monsters - 1d6+X
1 - Internal revolt - the soul of the monster revolts against being so digested, and frees itself, violently, from your stomach. Take d4 damage and vomit messily.
2 - Uneventful escape - the soul slips out your mouth as you attempt to consume it, and nothing happens.
3 - Temporary minor boon - some the soul stuff assimilates into your flesh, powering you. Gain the minor advantage and minor disadvantage of the creature for d4 hours unless specified otherwise.
4 - Temporary major boon - all of the soul stuff is assimilated into your flesh, powering you - gain the major advantage and minor disadvantage for d4 hours unless specified otherwise.
5 - Temporary near faultless boon - all of the soul stuff is assimilated into your flesh, and your soul successfully suppresses the negative aspect of consumption. Gain the major advantage and minor disadvantage for d4 hours unless specified otherwise.
6 - Permanent minor boon - the soul stuff of the creature is absorbed into your own soul. You are permanently empowered you with the creature's minor advantage, and it permanently curses you with its minor disadvantage.
7 - Permanent major boon - a total near total synthesis of the soul is achieved, and your own soul comes out on top. You are permanently empowered with the creature's major advantage, and it permanently curses you with its major disadvantage.
8 - Permanent near faultless boon - not only is a total synthesis achieved, your soul is able to weaken the curse it is being placed under. You are permanently empowered with the creature's major advantage and permanently cursed with its minor disadvantage.
9 - Total control of the ritual - Choose the result you want.
Examples of advantages and disadvantages;
Animated skeleton
Minor advantage - you just won't die - if your character is killed, fight on for d4 rounds before collapsing in a heap of bones.
Minor disadvantage - clerical magic is less effective - half any healing or half the duration of the spell.
Major advantage - half slashing and piercing damage as your flesh becomes less important for your survival.
Major disadvantage - as you are half dead, half any healing from any source (aside from consuming man-flesh and hp gain from gaining a level).
Ooze
Minor advantage - jellified skeleton - may crawl through gaps the size of your head as your shoulders dislocate and the bones bend.
Minor disadvantage - your flesh becomes ooze, losing coherence - your skin sags and bulges and you lose 1 charisma.
Major advantage - Take half bludgeoning damage as the ooze like flesh and supple bones can bend to take.
Major disadvantage - Move half normal speed as your muscles become slower, though no less strong.
Troll
Minor advantage - double any healing.
Minor disadvantage - you're as pretty as a troll, so lose 1 charisma.
Major advantage - 2 in 6 to regain d3 hitpoints every round of combat.
Major disadvantage - take double damage from any flame or heat, damage cannot be healed except with magic. You cannot abide holding a light source or being within 10 feet of it, and sunlight makes you uncomfortable.
Kobold
Minor advantage - nimble and lighter - do not set off weight based traps.
Minor disadvantage - weak - reduce damage done by 1.
Major advantage - sixth sense - sense danger coming, increase Awareness to a 5 in 6 or a 6 in 6 if already 5 in 6.
Major disadvantage - cowardly - must roll above a 10 to not attempt to flee when faced with combat, + wisdom bonus if party is obviously superior to opponents, + strength bonus if obviously inferior.
In addition, when you have a boon, be it temporary or permanent, your body shifts to resemble the creature you have assimilated with. Devouring a lion may make your eyes catlike and hair a rough main whereas devouring an animated skeleton will make you gaunt and pale, with eyes like sunken pits. While the first couple of rituals may be hard to notice, people will quickly begin to see the character's new chimeric nature. Adventurers who partake in that taboo communion are feared and hated, perhaps rightly so.
The next stage of the communion is, after all, the consumption of Man.
To consume the heart of a man is a far viler act, one which rebels against the fabric of the universe itself and its established order, and for which the sinner is always punished. However, the power it can grant is alluring. Note - eating a random peon can only gain you up to result 5 - more powerful corpses in the form of warriors, wizards and mercenaries are needed for 6 and up. The same bonuses (ritual, superior corpse and sacrifice) in the above ritual also apply.
Consumption of Man - 1d6+X
1 - Cursed by the gods - the fabric of reality revolts against your misdeed. Roll on Vile Traitor to Life additionally (that is, if you would already roll due to your accumulation of The Mark of the Cannibal, roll twice).
2 - Foul misdeed - Your stomach revolts against the manflesh. Take d4 damage and vomit messily.
3 - Terrible knowledge - The entire life history of the person you just devoured is seared into your brain. It may fade, with time, but the memory of your crime will not.
4 - Terrible vigour - The lifespan of the living sentient being you killed is taken within yours. Regain d8 hitpoints, and unlike every other source of healing, this can heal you beyond maximum hitpoints.
5 - Stolen skill - The muscle memory and instincts of the victim become, for a time, yours. Gain a bonus to whatever non-combat, non-magic skill that most defined the victim for d4+2 hours.
6 - Stolen might - The spirit and power of the victim become, for a time, yours. Gain either +1 to AC and +1 to AB or one use of a spell the victim had - GM decides.
7 - Permanently stolen skill - the central nervous system of the victim both literally and mystically fuses with yours. Gain a permanent bonus whatever non-combat, non-magic skill that most defined the victim, and roll on Vile Traitor to Life additionally.
8 - Permanently stolen might - your soul totally and completely dominates theirs, absorbing their life force and spirit forever. Gain either 1+ to AC and +1 to AB or gain once per day use of a spell the victim had - Gm decides. Roll on Vile Traitor to Life twice and advance the Mark of the Cannibal twice instead of once (Note - this may cause you to roll on Vile Traitor to life three times. This is intentional).
9 - Total control of the ritual - Choose.
Consuming a man, as we have said, curses you. The first time you partake in that utterly forbidden communion, gain the Mark of the Cannibal, somewhere. It is small and hidden, for now, but will not remain so. Every time you perform the ritual, you advance the Mark another step.
Mark of the Cannibal
1 - Gain a small, indelible mark on your flesh. It may take the form of a ritual scar, a weeping wound or a tattoo. It cannot be removed, by magic or by knife, and if it somehow is, it return in d3 hours somewhere vital - like your jugular.
2 - The Mark grows - it is now the size of your handprint, and cuts deeper into the flesh. Roll on Vile Traitor to Life.
3 - The Mark grows - it now covers an area roughly the size of half your torso, and open weeps. Small animals and children are uneasy around you. Roll on Vile Traitor to Life.
4 - The Mark consumes - it now totally covers your body everywhere but the hands, face and neck. Animals are afraid of you - so are children, but they do not know why. Roll on Vile Traitor to Life.
5 - The Mark now covers every inch of your body in one way or another. Everyone fears you, and rightly so. Gain +1 on every roll of The Consumption of Man table. Roll on Vile Traitor to Life.
6 - At this point, you must save vs Magic to not immediately attempt to consume the fallen, be they ally or enemy. Gain +2 on every roll of The Consumption of Man table. Roll on Vile Traitor to Life.
7+ You are now a creature of legend - the ghoul, the wendigo, the vampire. You lust for human flesh and no longer require or want normal food. The character likely needs to be retired, and will probably attack the party immediately or, if they were smart in life, retreat and attempt to ambush them. After all, the party is likely the best source of powerful meat around ...
Vile Traitor to Life
1 - You grow wicked fangs. They deal d4 damage and are initially easily hidden. You may regain d4 hitpoints by drinking the blood of still living being. If rolled again, increase the size and number of fangs and step up the damage die - this may be done multiple times. No longer so easily hidden.
2 - Your gaze becomes cruel and predatory. If rolled again, you may make an opponent save vs Fear with a hard stare - this takes up your turn if done in combat. If rolled a third time, re-roll.
3 - Your fingernails sharpen and lengthen. Gain a d6 claw attack, and if rolled again, increase the size of the claws and increase the damage die by one step - this may be done multiple times.
4 - Your movements become fluid - unnaturally so. If this is rolled a second time, gain +1 AC. If rolled a third, re-roll.
5 - Your legs snap and lengthen - increase speed to 1.5x human. If rolled a second time, your legs twist into digitigrade and you may jump twice the normal amount. If rolled a third, re-roll.
6 - Your noses lengthens and flares - gain the ability to smell living creatures within a 60 ft radius. If rolled a second time, your lust for blood increases in power and gain +1 AB against creatures which are already bleeding.
EDIT -
One anon on /tg/'s /osrg/ suggested restricting this to non-caster classes, which I think is an excellent idea. Casters have their own sources of magical power, and as they point out, it's very similar to the concept of hunting totem beasts.