You or your party is approached by a very scared, very nervous collector. He explains that as he was out hunting for rare herbs he heard screams coming from a cave in the mountain side. Upon investigating he saw large inhuman tracks leading out of the cave and inside he found mutilated corpses scattered about. Worst of all he saw these creatures that looked like severed human heads with bat wings where ears should be feasting on the corpses and turning the heads into one of their own. These nasty boogers are called Vargouilles ( Var-Gheel ). According to Volo's Vargouille are creatures from the abyss. They are incapable of reaching higher plains on their own but when a demon or devil is summoned to a higher plane they can latch on like ticks and enjoy a free ride to the higher planes. These things don't serve anything or anyone so their existence leave things wide open for you to toy with and open a whole mess of questions and investigations your party can have. If you don't want a band of cultists that just summoned a demonic entity then fine, don't the Vargouille have a pretty unique ability set that makes them fun and tricky to handle in combat, but, imagine instead of throwing them at your party you throw them at a nearby town... commoners would be no match against a swarm of Vargouille so your party would be faced with the oh so fun challenge of saving all the villagers! Of course if they decide to save all the villagers they would be known as heroes in that village and that is fun to roleplay as a Dungeon master/ Game master. Now imagine they save the village and in their celebration they are approached by someone that tells them "Thank you so much for saving our people adventurers but I have some bad news... I have read of these creatures before and if my studies have been correct, the only way they could have gotten to our plane is on the back of a much more powerful and evil creature when it is summoned" Boom new plot hook that involves cults and a gnarly evil thing!! There is so much fun that can be had with these things, I hope you find some creative uses for them. Thank you for listening and be safe adventurer!