Harmony Shoal

The Shoal of the Winter Harmony 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. (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. (TV: The Return of Doctor Mysterio)

History
The Shoal attempted to invade the Earth in the 2010s, using the Harmony Shoal as a front. They were defeated by the Twelfth Doctor and Grant Gordon, and subsequently apprehended by 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. (TV: The Husbands of River Song)