Fomorroh

"The Fomorroh will suck the life force out of you, everything that makes you Merlin will be gone, and in its place there will be just one thought. One thought that will grow until it's consumed you completely. One thought that will be your life's work. You will not be able to rest until it's done."

- Morgana The Fomorroh is a serpent-like creature of the Old Religion, used by High Priestesses to control a person's mind.

History
In the days of the Old Religion, the High Priestesses would use the Fomorroh to enslave the minds of their enemies. From Morgana's comments about summoning it from the depths, and it not being used to the light, its implied this creature dwells in the Spirit world. To call it in this world, Morgana cast a spell, ("Astige ðu wyrm fah ond geþéowe ðæt mod ðisse þeowes. Hine bind ond ða heold ond awendaþ he ealle") on a medallion on which was represented the Fomorroh and threw it in the fire. In her plan to kill Arthur, Morgana summoned a Fomorroh and used one of its heads to enslave Merlin, forcing him to try to eliminate the King. After several of his attempts on Arthur's life failed, Gwen and Gaius managed to catch Merlin. Gaius removed the Fomorroh and killed it, but a second one grew in its place. After stopping yet another attempt to kill Arthur, Gaius paralysed the snake's head, which temporarily gave Merlin back his free will. Under the guise of Dragoon, he traveled to Morgana's hut, found the mother beast, and killed it after prevailing in a close battle with Morgana. Back in Camelot, Gaius removed the head still left in his neck (A Servant of Two Masters).

Shortly before Morgana wakes up from her encounter with Emrys, she has flashes of him stealing the Fomorroh and defeating her in a duel, which results in a series of events to discover his identity (The Secret Sharer).

Appearance
The Fomorroh resembles a snake with a wide, short body and seven heads. It's knecks are long for the body's length, which is about half a foot. Fomorroh has greyish skin and yellow eyes. When infecting someone, it is still visible by its grey colour and shape. You can paralyse it for a concise period of time before it wakes up.

Abilities
As a creature of dark magic, the Fomorroh is a very dangerous being. Once placed inside somebody, it causes whatever instruction the person is given to become the only thing that they can think about and forces them to perpetrate whatever action they have been told to do. According to Morgana, the Fomorroh could suck the life force out of its victim and remove their current personality when corrupting a victim completely.

Victims of the Fomorroh retain their memories, but their personality changes, causing them to become rude and hostile to others, and obsesed about performing the mission. An affected person can temporarily regain control if the head, that has been placed under the neck's skin, is paralysed, but only for around 24 hours.

The creature is difficult to kill; even if one of its heads was cut off, another one would quickly grow in its place, and the disembodied head would stay alive. After Gaius had removed the serpent, it still grew back in Merlin's neck. The only way to definitively kill this creature is to kill the mother beast, by burning it in the fire and enchanting it with this incantation: "Ontende þisne wyrm þæt he licgeþ unastyred a butan ende."

Appearances

 * Series 4
 * A Servant of Two Masters
 * The Secret Sharer

Trivia

 * Body-wise the creature resembles a hydra, from Greek mythology, both in the number of heads and the idea of them growing back if decapitated. Unlike the hydra, it only grows one new head, instead of two.