Century Worm

The Century Worm is a rare giant creature that can appear in any part of the world. It's a mindless creature, uncontrollable, born to eat and moves across the landscape making deafening screeching noises. It's large enough to swallow a horse whole, and anything it eats will be broken down by more than stomach acid; the Century Worm's stomach carries millions of its young.

This creature is one of the more high-level encounters, where a character needs to be at Conan levels of strength and Merlin levels of magic. It's a pretty slow creature, so it's always possible to run away.