Mara

"It is the Mara who now turn the wheel. It is the Mara who dance to the music of our despair. Our suffering is the Mara's delight, our madness the Mara's meat and drink."

- Panna

The Mara is a mysterious alien creature which can manifest in dreams and possess the minds of others, controlling their bodies. When manifesting itself into the physical world, outside of a possessed host, it takes the form of a gigantic snake with long menacing fangs, and delights in provoking fear and chaos.

Biology
The Mara is sometimes referred in the singular as "he", and sometimes in the plural as "they", and appears to be a gestalt entity, created from dark thoughts coalesced together from multiple individual minds.

The Mara is only capable of crossing into the main universe by possessing a solitary mind, i.e. one that is not linked to a telepathic collective. After being banished to the planet Deva Loka, inhabited by the telepathic Kinda, the Mara spent centuries in confinement, unable to take over a host body and cross once again into the physical world.

Those who are possessed by the Mara manifest the image of a snake on their forearm; referred to as the Mark of the Mara, which can also be transmitted or duplicated to other beings, as the Mara can possess multiple hosts simultaneously. Hosts may also display red eyes, and their images in mirrors may be reflected with a giant snake skull in place of the host's head. The Mara is a highly sadistic creature, who takes delight in provoking fear and despair.

The Mara can be repelled by mirrors, as, apparently, it cannot stand to see its own image. Entrapping it in a circle of mirrors causes it to return to the world of dreams.

History
The Mara was originated on planet Manussa, whose people created a Great Crystal, capable of focusing mental energies. Unfortunately, as the Doctor put it: the Crystal absorbed what was in their minds, including the greed and restlessness, which was amplified, reflected, and brought to life in the form of a psychic creature.

Once created, the Mara caused the downfall of the formerly advanced Manussan Empire, which was replaced by the Sumaran Empire for the next three centuries. At this point, a representative of the Galactic Federation successfully banished the Mara, and Manussa was annexed by the Federation. Every decade over the next five centuries, the inhabitants of Manussa celebrated this victory.

Meanwhile, the Mara somehow ended up on planet Deva Loka, inhabited by the telepathic Kinda, whose joined minds prevented the Mara from crossing over to the physical realm. When the TARDIS landed on Deva Loka, the Mara invaded the dreams of a Human named Tegan Jovanka, and took over her body, from which it transferred itself to a Kinda named Aris. Luckily, the Doctor and the Kinda managed to entrap Aris into a circle of mirrors, forcing the Mara to leave his body and briefly display its real form, before being banished back into the dream-realm.

However, on the 500th anniversary of the Mara's defeat on Manussa, the creature revealed itself as still present in Tegan's unconscious mind in some capacity. Acting through Tegan, the Mara directed the TARDIS to land on Manussa and, despite the Doctor's best efforts, took complete control over the bodies of Tegan and the local Federator's son, Lon.

Once again, the Mara managed to return to the physical world by placing the Great Crystal back on its place inside the local temple, but the Doctor managed to use a replica of the same Crystal to concentrate his own thoughts and destroy the Mara once and for all.

Appearances

 * Doctor Who
 * season 19, "Kinda" (1982)
 * season 20, "Snakedance" (1983)
 * Tales of Trenzalore - "The Dreaming" (2014)