Redcap

Redcaps eesembled small, tough, and elderly human men or undersized wizened gnomes, standing around 3 to 4 feet tall and weighing around 50 pounds. They had hunched backs, wiry muscles, leathery skins, and evil smiles filled with wickedly sharp, protruding teeth. They wore heavy iron boots, leather pants, and the bright-red leather pointed caps for which they were named; these were always soaked in fresh blood.
Vicious and cruel, redcaps were insane serial killers who thrived on others' pain and dipped their caps in the blood of those they'd killed. They desired only to murder and cause carnage and mayhem, and acted only to satisfy these bloody impulses. However, their instinct to survive and need to soak their hats in fresh blood was the root of their murderous nature.
Favoring attacking head on, they had no sense of subtlety. Even when they tried to be stealthy, their heavy iron boots made them noisy and menacing, and clumsy except when rushing to the slaughter.
Redcaps needed to kill constantly in order to continue existing, absorbing a little of the essence of everything they slew. Their pointed red hats needed to remain soaked in fresh blood, which had to be replenished at least once every three days, otherwise the redcap would simply disappear, a fact they were entirely aware of. The more they did this, the more their physical abilities enhanced and the more resilient and naturally armored they became. It took a cold iron weapon to overcome their unnatural durability. An elder redcap, one that achieved great age and many kills possessed terrible strength and agility and was practically invulnerable.
Despite their small bodies and scrawny frames, they were actually fairly strong and relatively powerful, as capable as human-sized beings at swinging a large weapon or wrestling with a foe. They were fast for their small size.
Their heavy iron boots made them noisy, but not slow. They could kick a foe to much injury and knock them down.
When a redcap hurled a bullet from a sling, it glowed greenish-blue with an eldritch energy, and it struck harder than normal.
When it died, a redcap vanished completely, leaving behind just one tooth, which had magical power. It made whoever wore it be seen a little more favorably by other fey and especially by other redcaps.
Redcaps were natural loners, and rarely banded together. When they did, they did so only in odd numbers, forming small gangs of 3 or 5 or bands of 7, 9, or 11. Redcaps usually lurked within caves and old keeps and towers, usually abandoned after some tragedy or after they'd massacred any former inhabitants.
With hot tempers, redcaps argued and brawled with one another incessantly, usually to the death. They would slaughter each other without hesitation. It required an elder redcap, even more than one, to reign in their tempers and murderous impulses, mostly by bullying the youngers into submission.
Occasionally, a redcap would soak its cap in demon ichor instead of blood. This caused them to become an even more violent and volatile variant known as a madcap, who were known to incinerate upon their deaths.
Redcaps had a set of superstitions intricately interweaving with one another. In particular, they believed even numbers were unlucky, and thus would only carry odd numbers of coins and formed groups with an odd number of members. As easily as they killed one another, they refused to soak their caps in another redcap's blood, for fear of a curse. They believed rivers were home to water spirits that could steal their souls; thus, they washed themselves only in still water.