Ottoia

Ottoia is a priapulid, a primitive worm, that lives buried in sand up to ten feet deep with only its head showing. When the proboscis is pulled in, the head is a featureless, roughly round knob bristling with short spines.

This strict carnivore waits head-down in its hole, several feet below the opening, feeling for footsteps. When prey comes near, its proboscis shoots out as far as six feet to bite for 2d8. If the attack roll beats the needed number by four or more, the victim is swallowed whole; under the companion science-fiction rules this takes a Success Margin of 7.

A swallowed victim gets one try to break free, a Strength roll (or Fitness roll), and otherwise suffers 1d10 damage per round from digestive juices until dead. Attacks on the worm deal half damage to anyone inside, though with the body buried there is little to hit. Touching the spiny head inflicts 1d6, and in the science-fiction rules any attack on the head is reduced by three damage points and two lethality ratings.

Worse, ottoia usually live in rings of six burrows, so one bite often means the party has blundered into a minefield.