The Yellow Eel was a large water creature who lived in the ancient well of Kotir under Redwall Abbey's south wall.
When the wall was knocked down for repairs, Sloey accidentally fell down the well. The eel attacked, but before it had the chance to kill her, Skipper borrowed Tammo's dirk, leaped down the well and attacked the eel.
After a long battle, the otter warrior killed the eel, and they were washed farther down the well and eventually came out of a subsidiary of the River Moss.