Space crystal

A flock of flesh-eating, telepathic space crystals fell to Earth in 1842 and generated a telepathic field, as was their usual mode of attack.

Some of the crystals were made into a snowman by a lonely young boy called Walter Simeon, and instead of causing the snow to become a monstrous being that would attack Simeon, the telepathic field reflected Simeon's own dark thoughts. This gave birth to a parasitic consciousness which eventually learned how to live on without any physical substrate; after the Doctor, Clara Oswald and the Paternoster Gang destroyed the space crystals in 1892, the consciousness escaped, becoming the bodiless Great Intelligence which had already threatened the Doctor at earlier points in his timeline. (TV: The Snowmen)