As soon as he was close enough, Arthur grabbed Merlin's arm in a fierce clench. "What do you think you're doing? Gaius told me about this tide."
"Then you shouldn't be here! If it comes in we'll both be killed."
They watched the dark line on the horizon.
"I need to work," said Merlin.
"You need to come with me," said Arthur.
"You don't understand."
"I must certainly do and I forbid it." He frowned in irritation. He'd forbidden it the day before as well, and that hadn't stopped Merlin for one moment.
He tried a different tack. "Please."
Merlin shook his head. His lips were white.
"Don't do it," said Arthur. "I won't let you punish yourself for... Why are you punishing yourself?"
Merlin sighed, and glanced at the sea. "I'm not. We need to move."
"Why?" Arthur insisted.
The water surged towards them. Merlin saw horses leading the charge, their nostrils flaring, their coats pure white.
Arthur saw them too, and swallowed. But his nerve was strong. "Tell me."
"Gaius," said Merlin. He grasped Arthur's hand. "Now - jump!"
He raised his staff, and with a word, lifted himself, and Arthur, into the air. Arthur cried out, but the staff powered upwards, dragging Merlin with it, and Arthur, his hand in Merlin's strong grip.
Below, the world roared white fury. Rocks churned into the foam, and all that the tide touched, was swallowed. Arthur watched horses trampling water-fields, and saw other creatures too, forms like men at the plough, women with spindle or a swaddled babe. Some shapes Arthur could not have named, for they were only suggestions of life - an eye, opening through the surf, or a sudden gape of jaws. Monstrous ideas thrashed beneath the surface of the tide.
"Good god," said Arthur, and looked to Merlin, but Merlin had closed his eyes.
Still they soared, and Arthur, king of many battles on land, became afraid. It wasn't natural to be in the sky. "Merlin. Merlin!"
Merlin was still clenching shut his eyes. Arthur saw the glint of tears on his friend's cheeks.
"What's wrong?"
"The tide is falling," said Merlin, although he could not have seen. He lowered the staff, wrenching it downwards like a soldier struggling with a drawbridge. Arthur's stomach lurched, and dropped, and so did Arthur.
In moments, they were on land again, standing in soaked grass above the beach. The world was still as birds and beasts shivered in silence, far away from the magical flood.
Merlin's eyes were wet.
"What is it," said Arthur, embarrassed.
"Gaius," Merlin said again. "I didn't save him. And the tide will answer me but once a year."
Arthur floundered. Deception, and regret - or not those reasons at all. When had Merlin ever been ashamed of who he was? "You did this... for your uncle."
"He's ill."
"He's old," said Arthur.
Merlin went white. He clutched the staff and said, "Shut up."
"I'm sorry. But he is."
"He is my only family! I can't let him die!"
"Don't be bloody ridiculous! Swapping your life for his, that's not what he wants! He's an old man, he knows he can't have long -"
Merlin swung his fist and caught Arthur a sharp blow to the jaw.
Arthur sprawled on the ground, from shock as much as from Merlin's knuckles connecting with his face.
"Not one word," said Merlin, pointing the staff at Arthur. "Not one!"
Arthur thought of reminding Merlin who was king, but the look in Merlin's eyes told him no.
"Camelot needs Gaius," said Merlin.
"It needs you," Arthur said. "I need you."