"Before the Animus came, the flower forest covered the planet in a cocoon of peace."
The Animus; a.k.a. Pwodarauk, the Carcinoma or the Dark Power; is a huge fungoid entity, sapient but evil, which settled on the planet Vortis at an unspecified point in that planet's history. She has the appearance of a spider/jellyfish/fungus creature surrounded by a huge subterranean organic mass of growth. She is described as a parasite which grows a system of roots and plant-like structures around herself, penetrating into the core of the planet and feeding off on its magnetic energy, inadvertently causing the planet's orbit to attract new moons (and later the TARDIS) towards it. After she invaded Vortis the once temperate planet was turned into a barren landscape and the water was mostly turned into acid. Furthermore, the Animus took control over the native Zarbi and Larvae Guns and used them to wage war against the peaceful native Menoptera, causing the later to flee from their homeworld and settle on the moon of Pictos. A powerful entity, the Animus can control minds and even take control of the TARDIS to an extent, but appears to be unable to read minds – at least not the mind of a Time Lord. She can also create a web-like tissue to immobilize enemies, and possibly other structures as well. The Animus is ultimately destroyed by the Isop-tope device built by the Menoptera, which reached the center of the creature with the help of the First Doctor and his companions. It is assumed that the planet slowly regained its right vegetation afterwards.
A number of sources (of unknown canonical status) have acknowledged the Animus as one of the Great Old Ones from Lovecraftian mythology. The same treatment has been given to several other Doctor Who entities, including the Great Intelligence (identified as Yog-Sothoth), Fenric (identified as Hastur the Unspeakable), and the Gods of Ragnarok.