Doro no Oni

Doro no Oni is an oni that had the form of a puddle of mud. It was thus perfectly camouflaged in the Shadowlands swamps and gave quite a tense atmosphere to every patrol in those humid regions.

Attack
The Doro no Oni would attack when someone stepped into it. The mud would come to life, engulfing its prey, worming its way into the armor, the ears, nose and mouth. This would end in suffocation for the unlucky person which was attacked. Doro no Oni then would digest and absorb its prey, increasing its size.

How to defeat it
Cutting it would fail automatically, since Doro no Oni is a liquid creature, and trying to slice through it would most often wound the prey. However fire and jade were quite effetcive againts this oni.

Anecdote
A regiment of Hida bushi once happened upon a swamp near the River of the Dark Moon. Out of twenty men, eight survived, and they told that this whole swamp was a single Doro no Oni, stretching as wide as several square miles.