Harmony Shoal

The Shoal of the Winter Harmony (TV: The Husbands of River Song) - or, as they were more commonly known, the Harmony Shoal (TV: The Return of Doctor Mysterio, COMIC: Ghost Stories) - were a race that moved from planet to planet, taking over the bodies of the original population. (TV: The Return of Doctor Mysterio) At some point they were conquered by King Hydroflax, whom they revered fanatically and were sworn to protect. (TV: The Husbands of River Song)

Biology
The Doctor described the Shoal as "multi-nucleated organisms functioning as an infinitely adaptable, independently functioning peripatetic central nervous system". Physically, the Shoal resembled disembodied brains with eyes. (TV: The Return of Doctor Mysterio)

To move and interact with their environment, the Shoal appropriated the bodies of humanoids by removing the original brain and emplacing themselves. These "containers" gained a distinctive diagonal slash that split the head in two; the Shoal could separate the halves at will and hide objects inside. The internal structure of these individuals became blue and slimy. They appeared to be able to switch bodies extremely quickly as one was able to switch from the body of Dr. Sim into a UNIT soldier in the matter of apparently just a few minutes, all completely undetected. (TV: The Return of Doctor Mysterio)

History
The Shoal attempted to invade Earth in 2016, using the business Harmony Shoal Institute as a front. They planned to crash a spaceship into New York City, destroying everything but the Harmony Shoal building (which had been constructed in such a way that it was able to survive atomic bombs). This would cause authority figures and world leaders to take refuge inside the Harmony Shoal buildings, which had been placed in every capital city on Earth. The Shoal planned to replace their brains and then colonise Earth. They were defeated by the Twelfth Doctor and Grant, but one managed to infiltrate UNIT. (TV: The Return of Doctor Mysterio)

The Shoal intended to buy the Halassi Androvar from River Song to honour Hydroflax, not knowing that the diamond was inside Hydroflax's head. Scratch and other members of the Shoal met River aboard the Harmony and Redemption, though the sale was derailed after they found out where the diamond was. The Shoal members were killed when the ship hit a meteor storm and crashed on Darillium, killing all aboard except for River and the Twelfth Doctor. Unknown to the Shoal, River, who was also an archeologist from the future, chose the place and time of the meeting carefully so that it coincided with the meteor storm just in case she got double-crossed. (TV: The Husbands of River Song)

Behind the scenes

 * Their design of disembodied brains with eyes in a jar is similar to the Morpho, though they were not stated to be related.
 * Their ability to wear a body suit by opening them through the head is similar to the Slitheen. The difference was that the Shoal opened their heads by a slight touch to the cut on their heads, as opposed to the Slitheen who open the skins by a zipper just below the hairline.