"If the unseen stars are aligned just so on the night of a new moon, when rain pounds upon the surface of the ocean, the shape of a giant rises from the surf. A towering figure of coral, standing and striding along the shore."
"It is known that coral is an animal and not a stone, but its spirit is slow and its dreams are deep, awakening only rarely. When it does, it sends forth a coral colossus from its bed, to find a new place near the shore to lie down and sleep before day comes, seeding a new bed of coral."
"Coral giants have been known to tread through fishing villages and wharves in their waking-sleep, bringing ruin to docks and small vessels beneath their rock-hard feet. This is likely not malice; after all, how often do we knock asunder anthills in our walking, never minding the fragile dirt of which the swarms build their palaces."
"Reportedly, the fisherfolk of Whitecliffs have an old custom of lighting a large bonfire in the midst of their village for every new moon, with the stated purpose of "quieting the coral," a ceremony attended by quiet singing reminiscent of birdsong. I believe this may be a means of protecting such communities from the misfortune of an encounter with a coral colossus, who may be tricked by the fire-light into thinking the sun is close to rising, therefore returning to the sea to sleep and grow. Indeed, many coastal hamlets of Whitecliffs are surrounded by labyrinths of coral beds, imperiling navigation by ships larger than a sloop."
"It is written that the horde of one of the mage-kings of old had a giant of coral at its vanguard, but how even a powerful Magus of the ancient world would lure a colossus away from the shore and into war is beyond my guess."
- from the writings of Andras, Magus of the Fifth Order
Coral colossi are not supposed to be a foe to fight, but a puzzle (with a time limit) to solve. (Thus, I have not provided a statblock.) If player characters hear that a massive coral giant is approaching a nearby coastal village, or even larger town, it may fall to them to figure out how to stop or divert the threat. (My first thoughts are either tricking it into thinking the sun is soon to rise, as above, or perhaps providing a good solid rockbed near the shore for falling asleep upon. Or, you know, beating it up, but that is difficult considering its size and durability - I was thinking thirty or forty feet in height, but more wouldn't hurt if it would fit with the tone of your gameworld.) Coral colossi don't really think in terms of fight-or-flight, not having any natural predators, but might try to eliminate something that just badly wounded them. Or, y'know, wade into the safety of the ocean. Mission accomplished, in that case.
Coral colossi might just be an atmospheric tidbit, too. The grinding of salted rock and roar of broken waves as a massive figure moves against the dark horizon lets the players know their characters aren't just on a long walk on the beach.
(Edit: coral giants were inspired by the song above, "Colossus" by In Mourning, but it appears I am not even the first to put a coral giant in a game)