What D&D game is complete without harvesting dead monsters for their body parts?
The dreaded Troll Serum is one of those sick and twisted ideas I once sprang on my players, which turned out to be hilarious.
First, you kill a Troll, which can be quite tricky if the D.M. Is the kind who plays most regenerating creatures as sneaky, hit and fade predators who use their healing powers to best advantage..
Then you grab an end of it's severed intestine, tie a rock to a rope, ready a bucket, painstakingly thread the rope through the entire length of intestine, and then brutally drag the rock through after it, stripping out the grotesque fluids which line the insides of one of the most voracious eaters in the D&D monster world.
Take the bucket full of green slop and transport it back to your local alchemical lab, then get to work refining and adding the secret herbs and spices.
Once completed, the green and pungent smelling liquid is stored in a large flask, ready to sell to the next customer, I mean, you wouldn't want to try this stuff yourself, not for a second time anyway, and here is why..

Drinking the troll serum requires a high check of the character's Endurance (level appropriate, but it should fairly daunting) as the character slugs back an entire quarter of a gallon (or 1 Liter) of this vicious, acidic, slimy and extremely foul tasting potion.
The effects are not immediate, unless one counts nausea and a stream of constant gagging, retching profanity from the character as an effect.
After a few minutes, the character suffers stomach cramps and an itching sensation, with the first tell tale signs of serum intoxication, eyes that turn vivid green and an increase of +2 to the character's Strength attribute.
After four hours, the green hue starts to taint the skin of the character and they get a constant feeling of gnawing hunger, as the metabolism kicks into high gear. They gain Regeneration (one point per round) and +1 to Initiative checks.
The hunger becomes such that the character thinks of little else but eating if not otherwise occupied.

The side effect of the serum becomes a reality as the character now needs to eat at least a pound of meat per day, or they will suffer from vicious, immobilizing stomach cramps, fever and all the usual mental symptoms of drug addiction for at least six hours, before they eventually vomit up a vivid green fountain of bile and pass out for another six hours, no doubt vowing never, ever to do that again, once they wake up.

After a week, the regeneration effect increases to five points per round, the skin becomes a darker, very obvious green and if the character is reduced to zero hit points by an attack that does not involve Acid or Fire damage, they can regenerate from near death, recovering 15 hit points and getting back to their feet with a move action.
However, the requirement for at least a couple of pounds of meat per day will become the driving factor of the character's existence.
Two pounds of meat is a LOT of meat, and the character will more often than not only cook it to make eating it a little faster, they don't care what it tastes like.
The character will kill and eat just about anything to stave off the effects of the hunger, including riding mounts, pets, faithful animal companions, they will even strip carrion from a week old carcass, making whatever excuses and justifications they need to in order to assuage the demon hunger in their belly.
The withdrawal effects of the serum at this stage are twice as bad as before, putting the character out of action for at least a full twenty four hours, but, even if they think they are going to, or simply wish they were about to die, they will recover fully from the ordeal.
Thanks to the regenerative properties of the serum, this sort of abuse is possible, otherwise it would certainly be fatal.
Troll serum sells as a level 5 magical potion, at the bargain price of a mere 800 gold, and knowledge of how to make it is, unfortunately, fairly common.

Oh I should point out that a failed Endurance check means the character up chucks this potion, it has no effect (other than the nausea and stream of heart felt profanity) and was a waste of 800 perfectly good gold pieces.

Next Up: The Slightly Moving Rod and other malfunctioning items.