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"

Found in the “depths of the human heart”, the Mara is a creature of pure greed and hatred. It was accidentally created from human mental energy, amplified by the Great Crystal on Manussa, leading to a reign of terror. Banished to the Dark Places of the Inside, the Mara entered the mind of Tegan on the paradise world of Deva Loka. It was through her and a Kinda named Aris, that it found true physical form as a serpent. Despite being trapped in a circle of mirrors and banished once more by the Doctor, the Mara retained a hold on Tegan. It made her take the TARDIS to Manussa, 500 years after the Mara’s exile, and once more tried to manifest itself. After being trained by a snakedancer to find the “still point”, the Doctor was able to focus his mental energy to destroy the Mara. Although permanently freed of the Mara’s influence, the experience greatly affected Tegan.