Hekn Dng'Erndal

"It's out there. You see the coils sometimes, moving around the grass. Usually it's just a hint, a stirring of the soil. Doesn't even kill the grass. Sometimes, when the wilds are up and the beasts are on the move, you see the coils in a half-mile arch. Only ever saw that once, many years ago, but if I saw it in the Great Grass Ocean once, you can bet it's happened more often where no men are near to see. Never saw the crowned head, though. Probably a blessing."
~Abide Baht Mig, of Abide and Seak's Bizarre Bazaar

Summary

According to the old wives living on the plains, the Great Grass Ocean is home to a serpentine Ferneth known by many names. In the Old Tongue, its name is Hekn Dng'Erndal, which means He that sleeps in soulful dream. Great hope of many, king of many slumbers. The more common names used are "The Coils", "King Serpent," and "Sinkhole Sam." 
Legends say that The Coils are ten miles long, and nigh invincible to anyone who isn't a great hero. The head is rarely seen. It is described as a golden adder with a skull the size of a cow. Atop the head is a bony ring that looks like a crown. Two horns twist backward, occasionally sparking lighting between them. The venom of the serpent supposedly causes a permanent, mummifying sleep that can only be cured with some esoteric ingredient that changes from telling to telling.
On the rare occasions that The Coils attack, it usually does so to feed by opening its mouth just under the surface of the ground. The sinkhole will fall into the massive beast's mouth along with whatever poor soul was standing there. Once it closes its mouth and moves on, all that's left is a divot in the ground a few feet down, easily filled in by the following season.  Even more rare is the tale of great heroes who attempt to cross the Great Grass Ocean. When such a notable or revered figure sets foot on the plain, they garner the attention of the King Serpent. It will attempt to strike at them, exposing them over and over to its venom. Legends tell of an underground sepulcher where such honorable victims are stored in a mummified slumber, awaiting the ingredient that wakes them.

Variations & Mutation

Variations on the central story of the King Serpent fall along two lines: The cure for its venom and the list of its previous victims. If you name a great hero of history or legend, someone has claimed that they rest in the Serpent's Sepulcher, waiting for resurrection.
As for what can awaken these great men and women, stories vary. Some say it is the kiss of a true love, the blood of a descendent, a rare yellow flower (specifically what type is usually a vague mystery) or even the blood or venom of a basilisk.

In Art

The King Serpent fighting great heroes of the past is a popular motif, especially among artists on the Great Grass Ocean, where there is a lack of landmarks or impressive vistas.