The Unigawa

The Unigawa, or alternatively Vrotkop (meaning "Rot Head") were known to be a sanctum of shadow creatures, dwelling in different parts of Africa (originally), that wore the skulls or rotting heads of dead animals to impersonate them, gain their abilities, and lure wanderers to their death in the wilderness. Though this alone is horrifying, they could also bestow curses and call upon higher beings to protect them from humans who wished to banish them from their land. Vrotkop have been known to call upon the help of such beings as an Adze: a vampire in the legends of the Ewe people of Ghana and Togo. It takes the form of a firefly, but if you capture one, it will revert to human appearance. This can be dangerous in itself, because in its human form the adze may attack and eat your organs, but it can be defeated. However, in the insect form, the Adze will suck your blood while you sleep and spread disease. Its preferred victims are young children. The victim of an Adze becomes a witch who is possessed by the Adze’s spirit. The Adze ally with the Unigawa. One of the more powerful beings a Vrotkop can call upon, are Djinns: often perceived as wish granters without masters, Djinns answer to no one unless they want to. When a being is called by one of these shadow creatures, they by no means have to come, but the Unigawa will often present them with gifts if they do. They are sly and cunning, and will go to extreme means to get revenge. 

Once banished from your land, the Unigawa will harbor a grudge. It will spend the rest of it's life searching for a way to get back at you unless you make peace with it by gifting it a child in sacrifice; that child will die and become one of them, but the Unigawa will never torment you again and you risk no run-ins with higher beings.

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Appearances 

In the story arch Stinging Nettles, Elizabeth Sommer manages to make peace with a Vrotkop for a short time by means of trickery. After banishing it, it required a child sacrifice that Sommer was not willing to make. She cast a strong illusion, making the creature believe it had gotten what it wanted, but this only worked for a single day. The thing returned angry and with the aid of a particular Djinn. Pregnant at the time, she would suffer a miscarriage, which lead to the suicide of her husband who believed Elizabeth had sacrificed their child to the Vrotkop. Either way, the shadow creature gained it's victory and left.

Chapter one of In the Valley of the Damned, Sam Winchester accidentally contacts the Djinn that caused Elizabeth Sommer's miscarriage in a dream but quickly forgets about it, as it didn't feel like a vision to him. Meeting this Djinn in the astral realm was usually fatal to humans. Sam, however, woke up with just a headache. The Djinn realized that he survived because he had Terrenial blood in him. The Djinn now knew all of Sam Winchester's memories and feared the resurfacing of a potential world-ending scenario, so opted to try and extinguish this vessel from the Blackbloods' further use. The Djinn found possessing Sam very difficult with the protective ward of the tattoo encasing him, but he was eventually able to do it and tried to get Sam to kill himself. Sam's mind resisted just enough to prolong the efforts of the Djinn for Dean Winchester to save him. Though the stress of resisting a Djinn made him forget that he had done it altogether. Having survived the Djinn, the entity gave up and left Sam's body to rethink another strategy. Sam's memory failed him and he couldn't explain what had happened. The Vrotkop that plagued Sommer is eventually killed by the Winchesters in chapter two of In the Valley of the Damned.

The Unigawa that called this Djinn to exact wrath on Sommer, had given the Djinn an amulet of power in thanks for his help. This amulet originally belonged to Persephone, and held a living energy that was tainted by the depths of the Underworld. However, it restored life and masked the wearer from Celestial beings as well as Zeus in particular. This was why Castiel and Gabriel could not determine what possessed Sam. It seemed entirely foreign to them. This Vrotkop would later become an ally of La Bruja Roja in Stinging Nettles: The Hell House.