Shoal of the Winter Harmony

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.

Biology
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 hijakced bodies are usually referred to as either "vehicels" 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 "vehicels" head and hide objects, such as weapons or valuabels 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.

Culture
The Shoal are a hostile and invasive species, showing no signs of empathy for their hosts that get killed during the bodysnatch. The Shoal that inhabited the German scientist Dr. Sim even showed sadisitc tendencies while preparing Mr. Brock for surgery, making jokes about the procedure. Later the "hijacked" Mr. Brock showed similiar humor 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.