HOOTHI

The Hoothi are ancient communal parasitic fungoid lifeforms that lives off decayed matter and farms whole worlds, keeping sentient species as herd animals. Their sentience may initially have originated from their infection of a sentient being. Through the harvesting and assimilation of other species, they gained access to very advanced technology. They travel through space inside huge gas balloons.

More than 4 billion years ago, The Time Lords of Gallyfray were mediating in a war on planet Tranmetgura, hoping to organise a planetary govenment which they could interact with. The Hoothi arrived, infecting and animating corpses under the control of their collective consciousness and pushing them into the fight. The escalation of the conflict and the introduction of biological agents reduced the Planet's population by two thirds, which were harvested by the Hoothi. The gallyfrayans sent an ambassador to the Hoothi, but they used him as a host. Their attack on Gallifrey was quickly defeated by superior biochemistry. The Hoothi fled into Hyperspace before the Time Lords could timeloop them.

Circa 1,000,000 BC, the Hoothi returned. On the planet later known as Heaven, the gave the natives the ability to turn their world into a beautiful garden world. In return, the Hoothi harvested part of the population periodically in an event called the "Resurrection Dance". Believing the Hoothi to be their gods, most heavenites wanted to be in the half of the population that went with them. Each time it happened, the Hoothi showed the Heavenites how to make their world better. One of the last generations of Heavenites used Hoothi technology to build an observatory, chaining herself in it when the Hoothi arrived for the final Resurrection Dance and leaving a message on the walls to warn future generations about the Hoothi.

Source : Doctor Who “Love and War” novel, History of the Whoniverse site.