The Greth

The Greth was a unique entity native to the planet known as the Greth-Sphere. A sentient fungus, the Greth claimed the title of Evolution's Champion after becoming the sole lifeform on its home planet. Intent on conquering more of the universe, the Greth tried to take over the Earth on several occasions in the 20th and 21st centuries, becoming a pawn of the Yssgaroth in the process and spending some time impersonating the businessman Henry Bildbriain.

Biology
The Greth appeared as a mass of light green, fungoid mass. It was a single formless living organism, capable of spreading out its physical form or even splitting itself into smaller, independent chunks. The Greth, or pieces of it, naturally acted like a thick liquid, capable of being stored in vats, but blobs inhabited by the master-consciousness could shape themselves according to the Greth's will, for example growing tentacles to manipulate objects, or adopting facial features like two glowing lime-green eyes to more easily communicate with humanoids.

The Greth was taught by its patron to possess other living beings, taking them over from the inside and gaining access to their memories and skills as it overtook them. However, during its confrontation with P.R.O.B.E., the Greth admitted that it could only possess a limited number of humanoids at a time in its present state, needing to gain more powers if its dreams of merging with Earth's entire biosphere were to be fulfilled. (PROSE: Out of the Box)

Origins
The Greth originally evolved on a planet known as the Greth-Sphere. After entirely taking it over, becoming "Evolution's Champion", it decided to try and take over the rest of the universe. Splitting off chunks of itself into "spores", it launched them into space in the form of small "seed-meteorites", bound for other planets. It conquered many planets, notably becoming the "scourge of the Wizards of Ertshea"; although many were inhabited, others were "barren orphan-worlds". It once tried to conquer the Earth, but was defeated by a man with an umbrella and his cat. (PROSE: Out of the Box)

Banished into the Void
Some way into its interplanetary conquests, the Greth ended up on the planet Gallifraxion Four, which it found had already been conquered by a powerful species, the Clock-People. Taking admiring note of how the Clock-People had welded themselves to the planet's biodata to make it appear to any non-time-active observers that they were native to it, the Greth failed to defeat them and was instead captured. It was placed on trial for interference by the Clock-People's leader, and banished into the Void Between Worlds.

There, it was contacted by another entity, who had been active in the universe in the days before history, (PROSE: Out of the Box) the Yssgaroth. (PROSE: Preternatural Nights) Deciding to take the Greth under its wing, this "patron" taught it more about how to corrupt and overtake living things instead of simply destroying them. After telling the Greth how to recognise its agents, the entity then helped the Greth return to Earth through the Infernal Depths. (PROSE: Out of the Box)

In France
Emerging from the ocean, the Greth made its way to the shores of Le Touquet, a city in France. Its appearance was investigated by the Conservatory for Research on the Unusual and Xenogenic, or "C.R.U.X.", a French organisation needing with supernatural and alien threats. It tried to use its newfound powers of assimilation on the team's pet, Benny, underestimating the inherent biodata stability of cats. Unharmed by its attack, Benny lashed out at the Greth and tore it to shreds, with its remains carried out at sea by the tide. C.R.U.X. knew that it had survived, and would make its way to England, but decided that the Greth would now be the problem of their British counterparts, the Preternatural Research Bureau, and were thus no longer their concern. (PROSE: Out of the Box)

As Henry Bildbriain
After arriving in England, the Greth managed to kill London-based millionaire entrepreneur Henry Bildbriain, of Bildbriain Dairies, and absorb his memories. It then filled out Bildbriain's skin and impersonated him. Firing most of the staff of the company's factory in Woolwich, the Greth then began to fill vats in the factory with its own substance, its aim being to insert this fungus (instead of the ordinary edible one) in blue cheese sold under the Bildbriain brand, tricking thousands into ingesting part of the Greth and allowing it to take them over. However, before it could do so, it needed to experiment with holding on to many bodies at a time. It used the factory's last two workers, Jacks and Bottaro, as test subjects, but both of them died; the story got out and the Greth, in the guise of Bildbriain, had to make a statement in the papers, claiming their deaths had been caused by industrial accidents. (PROSE: Out of the Box)

Defeated by P.R.O.B.E.
A few weeks after the deaths of Jacks and Bottaro were reported, Giles, the director of the Preternatural Research Bureau, realised there was something strange going on at Bildbriain Dairies — something Box had realised some time prior. He decided to investigate there undercover, disguising himself as "Mr Gilbert Slige" and taking Maxie Masters (transporting Box), Agamya Akhtar, Azacca Dixon and Tasha Williams with him. The Greth, as Bildbriain, let them in while under the impression that the groups were agents of its patron, but, after it ascertained that they were not by catching sight of their reflections in a mirror, led them to the factory's main room and dropped its human disguise.

It kept the group alive, hoping that one of their bodies might be a useful one to take over, long enough for Maxie to trick it into letting her connect Box to the Internet by claiming that Box was a "data-cube" containing the Sun Builders' secrets of ubiquity, the very thing the Greth desired. Using superhumanly-fast hacking, Box called various groups and individuals to the factory at once, forcing the Greth to evacuate through the sewers. Box reassured the P.R.O.B.E. team that she had foreseen this as well, and had put in a request with sewage inspectors for all waters from the Bildbriain factory to be automatically boiled, thus hopefully ensuring the Greth was finally and completely destroyed. (PROSE: Out of the Box)