The Keeper of Sacred Law

When you walk the forests of my homeland, be wary, friend. Though there are few places as deep and dark as the woods around here, ours are home to a guardian of sacred places, and they don't take kindly to people pushing too far into their territory. You may not even see it coming, despite it's monstrous size. It can appear as clearly as a crystal stream, and the only sound you'll hear before it ends your life is the sound of running water.
The Taniwha sleep, as long as you leave them to sleep. May the Mana Atua save you if you happen to wake one.

Summary

The Keeper of Sacred Law refers specifically to a monster spoken of in the legends of Kohoruna, named the Taniwha. It starts with a Toa'ana fisher finding a bountiful river in the jungle, where they catch an incredible bounty. Before leaving for home, the fisher is confronted by Moawhaea (the Monumental of the Sea in Toa'ana culture) who requests that the fisher not abuse the gift of the river. The fisher agrees, and the river is declared Tapu (or sacred) in the presence of the Monumentals. But the bounty is too great to ignore, and the fisher returns the following day. And the next. And the next. Until the fish of the river seem to have disappeared entirely.
Disappointed by the catch, the fisher rises to leave, before the waters of the river seemingly rises alongside him. The liquid engulfs the fisher, changing them from a jade skinned Toa'ana into a serpent bodied, four legged beast, all the while the promise to not abuse the river is repeated over and over in the fisher's mind. The next person to happen upon the river was amazed to see such a bounty just waiting to be caught. As they get up to leave, the water begins to shift into this serpentine being, and this time, it bears a warning: "Do not abuse the gifts of the river, or incur the wrath of the Monumentals. I am the Taniwha, and I exist to keep the sacred laws of this land."

Historical Basis

Whether the specific event that is depicted in the myth ever happened is debatable, but the existence of Taniwha throughout Kohoruna is widely believed to be true, and sightings tend to be centered around landmarks of cultural and natural significance. It is unclear if the creature is one created from the elements by the Monumentals, or if they are always mortals bound to protect these sacred sites, but common knowledge seems to indicate the latter.

Spread

Few outside of Kohoruna are aware of the existence of the Taniwha, but the myth is widespread on the continent. Village elders are quick to pass on their knowledge to young Toa'ana, and among their words of wisdom are warnings of the Taniwha and the sacred lands they protect.

Variations & Mutation

Certain recounts of the tale frame the beast as one of pure elemental fury, detached from the gods and acting as a pure monsters, while others tell of mortals bonded into service for the gods after overstepping their bounds. All seem to indicate the Taniwha's affinity for water, with larger monsters described in tales that take place out in open water.

Cultural Reception

Taniwha, whether seen as guardians or pure monsters, are believed to frighten off threats, so posts set at the gates of many settlements feature carvings of the creature. These carvings also find a home in shrines to Moawhaea, Monumental of the Sea and Birth, as she is usually associated with them.