Leech of Yoh-Vombis

The Leeches of Yoh-Vombis are a peculiar Martian species found in the underground ruins of Yoh-Vombis, built by the extinct Yorhis, and believed to have been responsible for their extinction.

Biology
These leeches are black, parasitic creatures which attach themselves to the skull of a living host and start feeding directly on their brain. Their extraordinarily sharp teeth and powerful digestive fluids quickly consume the entire top of the host's cranium and central nervous system, resulting in a quick and grotesque death for the host, whose corpse is then controlled by the necromantic leech. Their appearance at this point is reminiscent of a bloating black cowl covering the zombified corpse's upper head, brow and eyes while feeding on its internal fluids. They have neither head nor tail, and their tough skin is covered in fine hair which resembles a type of mold. On the underside, they have multiple rows of pinkish suckers arranged in circles around a pale disk, covered in nerve-like filaments.

The Leeches of Yoh-Vombis may also be extraordinarily long-lived, resilient, and possibly capable of entering some state of cryptobiosis or hibernation, as demonstrated by the large number of them which remain alive sealed inside the abandoned catacombs with nothing to sustain their population as far as it's known. They can survive independent of a host, and in this form they gather and crawl together in great numbers, forming veritable armies which move with surprising swiftness, considering their vermiform nature. Their foremost weakness is that they can't stand the sunlight, and must remain in darkness.

Appearances

 * "The Vaults of Yoh-Vombis", by Clark Ashton Smith (1932)