The Hellion curse isn't something that is casted onto someone, nor it something can obtain from another person. The Hellion curse is grown inside a human body. This curse is a seed inside the human body that grows larger and larger with the more negative emotions the person feels (ex; despair, hatred, anger, sadness, hopelessness, etc..) once the seed gets big enough the human body has a seizure-like attack until the soul leaves the body with the seed, the seed then hatches and becomes a monster called a Hellion.
Hellions are horrible creatures. They can hide themselves in different ways. A field can be placed to make them invisible or they can morph into somebody to look normal. Hellions can morph into anyone that the memories of their former body held, though there is no recollection of anything that that person might have been or felt or knew. Hellions only know how to trick and eat, they eat human flesh. Each Hellion has a gift associated with their former body, example; fire, healing, duplication, etc..
Anyone near a Hellion can be effected by certain emotions, such as anger and sadness. Hellions often like creating fights between close people or making suicide happen to depressed people, this is the Hellions idea for fun. Hellions have no actual mind like a human, all they know is that the want something and they will get it. This something is usually food or fun. Most Hellions can feed off of negative emotions from other people as well, if it is strong enough, which is why the like creating havoc between people.
Since Hellions are no longer the person they used to be nor will they ever be, they should be killed immediately. There is no getting their former person back, they are now a monster. There are Hellion Hunters, also called Enchanted Defenders, that defeat these monsters.
Hellions can be caused by anything. A break up, a fight, being alone... it is a tragic tale. Every person has a seed inside them and can not go back once they've turned. Their former body will remain limp and lifeless while their new form takes place. Hellions have a true appearance which is monster like and anything but human, each are different than the other and there's usually not many similarities.
Gleaners create them for various reasons but mostly to help the universe, themselves or for fun.