"Winds of Change"

Prompt: What's the Catch?

545 years After Founding
One year here is four times longer than Earth’s…
Strange how time suddenly had meaning to the dying elven king, wasting from an illness none could cure. Healer’s Grove in Heartholt held many such patients, most humanoid, but all residents of the forest metropolis or surrounding towns expanding the colony. Not Black Death, which had driven them to this world, but something native.
A unicorn healer approached his bed, “The others are here. Can you stand?”
The old elf tried, but failed, the mare catching him. He whispered thanks as she knelt, letting him ride. Living trees coaxed with magic formed walls and roof of the hospital, summer’s yellow-greens, and purples dappling the grass.
Others nodded in respect as he passed. He’d been infected helping heal here, or so they thought. None knew for sure how the sickness spread, though not airborne.
The unicorn brought him into a secluded room. Representatives from other stricken races ringed it, along with the Head Healer, an ancient feathered serpent.
“What news?” The elf’s voice was weak. He trembled, exhausted despite the gentle ride.
“Storms gather at the colony’s edges,” said the selkie, “Odd, but an effective boundary against the lethal wasteland beyond. They move outward with the magical barrier, and that sea is thawing now.”
“The Awakened animals are integrating surprisingly well under dragon rule,” said the centaur, “Hopefully peace lasts.”
“But still no cure for this blasted disease killing us all,” growled the dwarf.
“Actually–” piped the gnome, or the halfling?
“–that’s why we called,” finished the halfling–the gnome? Even side by side, they were confusing
“We’ve developed a potion…,“ the feathered serpent began hesitantly.
“What’s the catch?” asked the elf king’s son.
“It’s a polymorph, granting you immunity–but we don’t know what you’ll turn into. Or how much of yourselves will remain,” the serpent bowed his head, “There’s no other way.”
The dying king broke the stunned silence. “I’ll test it.”
“Father–?!” his son protested.
“It’s alright,” he said gently, voice gaining strength as he straightened on the unicorn’s back.
The mare looked back at him with a sad, proud smile.