The Shoal of the Winter Harmony were a race who were conquered by King Hydroflax, whom they revered fanatically. They resembled humanoids with their heads split in two by a diagonal slash; they could separate the halves at will and hide objects inside.
The Shoal intended to buy the Halassi Androvar from River Song to honor 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)