Note that this Monster Monday entry contains spoilers for the Clark Ashton Smith story "The Vaults of Yoh-Vombis." If you haven't read that story, I cannot recommend it highly enough. It is one of Smith's best works - some would say the best - and it along with "The Tale of Satampra Zeiros," "The Tomb-Spawn," "The Weaver in the Vault," and several others are very adaptable to D&D.
No. Appearing: 1d6 (4d6)
Armor Class: 7  <14>
Hit Dice: 2
Attacks: see below
Damage: 1d6 (automatic)
Hoard Class: XX
Vortlups are a horrid race of fungus-things that brought doom to a Martian civilization that was born and died millennia before the Martians known to the Uzites. These malevolent creatures made their way to Earth on Martian ships in the time after the Deluge, and they have remained here ever since.
These fungal-horrors appear as small sheets of night-black moss. They move in a manner similar to that of an inchworm - folding unfolding rapidly and at an unnatural speed. They are capable of crawling across walls and ceilings just as easily as the ground, and indeed they prefer to do so, though those that have just fed must stick to the ground and move at half-movement.
The vortlup attacks by dropping on its victim, making an attack roll to see if it lands on the victim’s head. If it does so, it will deal automatic damage each round until the victim is slain. This is how the creature feeds, growing bloated and filling with blood, hair, and the gelatinous residue of melted bones. Creatures slain in this manner immediately come under the control of the vortlup, which uses them for their own nefarious purposes. Such bodies are only usable by the creature for 3d6 days, after which point the body is too decayed to be propelled in any fashion. Vortlups will typically remain on the victim until another passes by.
Killing the vortlup will cause its grip to slacken and its cowl-like body to slump to the floor.
Creatures that manage to survive being attacked by a vortlup must make a saving throw or become the victim of a poison used by the vortlups. This poison infects the brain and causes the victim to perform whatever actions the vortlups desires. It takes effect regardless of whether or not the vortlups is still alive. This is the same chemical that vortlups use to pilot the bodies of the dead.
An ancient Martian chronicle states that the vortlups were originally created by a race known only as the Necromantic Ones. The precise nature of these beings is unknown, but they seem to have survived from the dim antiquity during which the solar system was born. While perhaps more common on Mars, Necromantic Ones are believed to exist on Earth as well, and the vortlups chief goal seems to be their reawakening. To accomplish this, they lead their victims to strange vaults in order to perform rituals best left to the referee’s imagination.