The Muscle Flies are creatures of the warm swamps and the tundra in its rare warm summer months. These foul and detested creatures look very like giant black and yellow mosquitoes, the size of a human head, and act very like them, preferring to land on sleeping humans and animals (although when they get hungry enough they will attack targets when they are awake. A thin ridge of hard bone runs down their back-this can fend off an attack by an edged weapon if it comes down on it, but a blunt weapon will shatter it.
Their needle sharp jaws inject three substances into their victim. The first is a numbing anaesthetic. If the victim is asleep this will keep him or her asleep for a while unless shaken awake, but will generally have run out by the time the sleeper awakes, rarely leaving a trace behind. On an awake victim, it can temporarily numb a limb although it will wear off within an hour.
The second dissolves muscle and/or fat, enabling the Fly to suck up the fat or muscle like it is blood.
The third is a healing substance that heals the wound. The Flies will generally not kill a victim except in self defence or when enraged, as they can only kill and drain a victim once, but they can drain a living victim several times if they don’t take too much at a time.
By night they act alone but by day they fly in packs of four or five, working together to subdue large and dangerous prey. Muscle bound humans have been left as weak as young girls by these things, as they prefer to eat human muscle. Many a muscleman has been rendered helpless as muscles made strong through years of bodybuilding are rendered down until they are left just strong enough to enable him to use his limbs.
If captured and manipulated by trained medical personnel these things can be used to drain excess fat instead of muscle. If starved enough they can be used to extract gangrenous flesh from a wound (they normally would no more eat rotting flesh then we would eat bad food, and as such shun the bodies of all but the most recent dead.) If these are seen feeding on a dead body then the body is has at most been only dead for an hour.
Since they so rarely kill, these can be used to punish people who have been judged to be using their muscles for crime.
It is possible (unless killed) to recover from these things, but it takes several weeks or months of exercise and lifting weights.
Additional Ideas (1)
The well-known, and highly quirky, alchemist Rennixx once cured the renowned warrior Talsmosh of a Muscle Fly bite; partly from professional curiosity, but mostly because of the hefty sum of gold the warrior pushed his way.
This cure required some of the squishier parts of the bug (some glands, a peculiar organ that heals the victims, and a few other odds and ends), ground up bone ridge (also from the fly), milk of a mountain goat, powders produced from earthworm castings in a valley of the Grannesti Mountains, and 7 specific herbs gathered in the Fallanks Wood.
Only the bug which had originally bitten Talsmosh could be used for this cure. The party which Talsmosh hired to bring back the beast decided it would be simpler to bring back the corpses of all six from the nest. Fortunately, the correct one was found after only two failed guesses.
Talsmosh was quite relieved once his strength had been returned to him, although he spent the next week in a tavern trying to drink away the taste of the cure…