A misnomer. The coral lives. The symbiotic algae inside, filling it with strange fires and bloody lusts, does not.
We have to blame Atlantis. As with so much. The long-time-dying, depthlessly mad, endlessly growing empire that crinkled on the planets skin like a fractal scar, always hungry. You know how they end. This is how and why they start, with blood, and bone extracted from the sea.
Their power grew, not through conquest, culture, or force of arms, but murderous biogenesis.
The empire was a pin-prick once. Long ago, one of a tidal archipelago scattered in a forgotten sea. Atlantis had few neighbours, no resources, no trade and nowhere to go.
Many coral and anemones cradle in their core a form of symbiotic Symbiodinium. An algae. It feeds the cradling beast with photosysnthetic skill and in return is guarded by the polyps care. The simple island-dwellers to the coral sang. They made it grow. Forming tiny atolls for canoes and spearing fish. Atlantis was a salty garden the size of St Helena. But the sun to them was moonlike and pale, eclipsed by natal imperial pride. The urge to empire outstripped the corals natural growth.
They spiritually and physically ruined the Symbiodinuim algae. Hollowed out its photosysnthesis and left its cytoskeleton covered in the tattered remains of the cell membrane, its mycoplasmal engines animated not with light, but with death. The ATP from animal cells drove the polyps into a mad, hollow simulation of ferocious life.
It grew and it grew fast and wild. Atlantis stretched out a bony beckoning finger of reef. It touched its closest friend. Then went to war. Not for land but blood. For Atlantis blood was land and land was strength. The future. The ultimate resource from which all others sprang. The source of human power. Ultimately, one island-culture stood victorious. The captives went to feed the hungry coast. Atlantis grew. It needed to keep growing. The island went looking for prey.
All of this took a couple of thousand years and multiple cultural shifts. But at the end, an isolated island state was transformed. Instead, sub-continent of sorcerer kings. New lands, by nature never planned.
Over time the ecosystem around Atlantis adapted to its predatory shore. Vast whale-consuming anemone bloomed in the coral shelf as it loomed. Mile-long jellyfish thronged under the surf, trailing gossamer neuro-toxin tendrils. Nothing survived the seas around Atlantis.
And then it fell. Atlantis shattered and drowned. The coral went un-fed. It did not die. Some flung fragments went wandering under the waves. Some though, was folded under the earth. Hidden in the stone. Waiting for the slow collisions of continents to cut it free.
Zombie Coral can lurch out of an old seam, waving sessile gorgonian fronds. It can form man-shaped things that stumble out of the dark with anemone hands. They are child-sketch-drawing-men. Perhaps the memory of the blood informs them. Perhaps they were men once. The slightest graze of their nodular pipe-cleaner limbs is dangerously toxic.
Desperate hunger and Atlantian science forced the growth of rare and murderous nematocytes holding saturnury ammonium compounds, proteeth, tiny-hydroxy-tryp-the-men, catch-you-cholamines see-me-no-more and hista-mind. Their touch is poisonous and paralysing. The delayed effect, weeks after the initial graze, is worse. Slow transformation.
Where did you think brain-coral came from? Its human brains, changed and growing, falling from the sunken skull.
Wounds should be flushed with large quantities of vinegar or whatever sterile fluid you have available.
The man-shaped coral things dimly recall an imperial dream. They can smell magic users and will flock them. The best escape is courtly grace. The coral knows the ways of old Atlantian times. If you should know them too, it may be duped.
Any magic-using PC who has learnt the complex courtly greetings and careful social rituals of ancient Atlantis, may perform them. No actual magic is involved. It is simply a form of unique and flowing approaches and moves. Precisely employed. Part tea-ceremony part dance. This is how high-cast Atlantians held above the mob. If done correctly any coral beings witnessing will cease attack. Lost in a memory of their greatness. They can be directed. They do not understand words or complex physical directions.