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The Shoal of the Winter Harmony was a hostile alien species in Doctor Who. They served as a supporting antagonist in the 2015 Christmas special The Husbands of River Song and the main antagonist of the 2016 Christmas special The Return of Doctor Mysterio.
The Doctor describes the Shoal as "multi-nucleated organisms functioning as an infinitely adaptable, independently functioning peripatetic central nervous system". The creature resembles a human-brain, though they have eyes that flash occasionally.
In order to move, the Shoal require bodies humanoid of humanoid species (such as humans). The Shoal replaces the brain, killing the original being in the process. The hijacked bodies are usually referred to as either "vehicles" or "containers".
A humanoid with a Shoal in it's skull can usually be recognised by a diagonal surgical scar, which the Shoal can use to split it's "vehicles" head and hide objects, such as weapons or valuables inside.
The internal structure also of the host body also turn blue and slimy, and sometimes they also bleed a blue fluid through their eyes, noses and ears.
The Shoal are a hostile and invasive species, showing no signs of empathy for their hosts that get killed during the body-snatch. The Shoal that inhabited the German scientist Dr. Sim even showed sadistic tendencies while preparing Mr. Brock for surgery, making jokes about the procedure. Later the "hijacked" Mr. Brock showed similar humour while trying to replace Grant/The Ghost with another Shoal.
Dr. Sim also claimed to the Twelfth Doctor that "the humans had no plan", presumably claiming that there would be nothing wrong with exterminating and replacing an inferior species.
At some point they were (presumably brutally) conquered by King Hydroflax and started to revere him in a fanatic way and were ready to live and die for him. One member called Scratch was even ready to offer his own head for the King.
The Return of Doctor Mysterio
From 2011 to 2016 the Shoal attempted to take over the world. They posed as the Harmony Shoal Institute, an international research organisation with an office in every major city in the world. The Shoal planned to crash a rigged space ship into New York, destroying the entire city save for the Harmony Shoal Institute office. As the world would fall into panic, the world leaders would seek sanctuary in all the other Institute offices around the planet. They were stopped by the Twelfth Doctor and Grant Gordon.
The Husbands of River Song
At some point after this they were conquered by the cyborg King Hydroflax in a violent manner, which led to them revere him.
Thousand of years later another group of the Shoal were on board the star cruiser Harmony and Redemption where they planned to buy the Halassi Androvar (the most valuable gem in the Universe) from River Song and the Twelfth Doctor. This group worshipped King Hydroflax and stood beside him when later appeared. They presumably perished alongside their king when the cruise crashed into the planet Darillium.
- The two groups encountered were quite different. The one onboard the Harmony and Redemption were quite fanatic and cult-like, while the ones attempting to take over the Earth seemed to be more rational.