Sea Pillars

The pillars are of varying heights. Once uniform, they have been worn down by the briny sea water. Each pillar is made of carved marble, covered in mottled green algae. Just visible beneath the growths are intricate relief carvings depicting horrendous beasts and demons.
Writ in large glyphs and wards between the horrendous carvings are many spells of warding and protection. Almost all of these, however, are corrupted and polluted by sea water. Or defaced by other things.
The sea floor around the pillars is littered with the sad remnants of creatures from the world above who dove too deep trying to read the strange words.

Long ago, when the world was still new, and men still knew mystery, there rose a great empire called Dradarrad. The men of Dradarrad were bold and strong, armed with biting iron and armored in strong plates. With a will like a sweeping fire they spread their lord's rule over the land; burning, pillaging, killing, enslaving.
Their lord was a chilling man with piercing eyes and a matching sword. He was feared in twelve lands and wanted for murder in three more. Their lord was Thadra Grundegg, Son of Shetesh. He lived for the thrill of the hunt and the rapture of the kill.
Thadra sat high on his throne, made of purest gold, so say the stories. Below him in his kingly hall, slaves and goods were brought before him as tribute by his soldiers.
But his tale is better saved for a different time.

What is of import is what he constructed in that long-forgotten valley near the mound named the Gellaman's Haed. For when his men brought him news of the great treasures that lay inside of that thrice-cursed tomb, Thadra made haste to go and see this grand prize. When he arrived, the dim horror that dwells in that place had already come out into the sunlight and wreaked havoc upon his men.
In great anger, Thadra brought together his most arcane sorcerers and called upon them to bind that dark thing back into its hell-hole.
The great sorcerers told their lord that before they could accomplish what he asked, they would first need certain foci for their spell. Thus, the great pillars were built.
After the spells were cast and the beast bound, Thadra and his entourage left the site, never to return.

As the years passed, those who lived near the pillars gradually forgot who built them and why. Stories grew up about the gods that made the pillars, and even those passed out of memory.
Next came the fairy tales that said that the pillars were a more permanent version of the faerie rings, where the fair folk would dance.
But then the valley filled with water from one too many rains, and the pillars were lost to sight.
The pillars' valley changed again in later years, the water eroding hills and ridges, forming a great sea, after a time.

Now, things of the deep inhabit the corridors between those tall standing stones. Great eels and serpents and some things that have not seen the light of day for so long that none remember what they are called.
Those of the glyphs and wards that are still active can still be set off, but not by such slimy creatures that stay near the sea floor where the stones have all been defaced.