Curse of the Unicorn

"When you killed the Unicorn, you unleashed a curse. For this, Camelot will suffer greatly."

- Anhora to Arthur

The Curse of the Unicorn was an enchantment that befell Camelot after Prince Arthur Pendragon killed a Unicorn during a hunting trip.

The curse caused Camelot's crops to die, its grain to rot, and its water supply to turn into sand. According to Anhora, the curse could only be lifted if Arthur passed a series of tests and proved himself pure of heart, after which Camelot's water and crops were restored.

History
The curse first manifested in the death of Camelot's crops and edible plants. The city's water supply turned to sand soon afterward, leading Uther to issue a city-wide curfew and decree that any looters would be executed.

That night, Arthur and Merlin saw someone out after curfew and chased him into the grain stores. The man introduced himself as Anhora, the Keeper of the Unicorns, and explained to Arthur that the curse would only be lifted if he made amends by passing a series of tests. If he failed, Camelot would be doomed for all eternity.

Arthur passed the first test when he caught Evan stealing grain from the castle stores. Though he had been ordered to execute all looters, Arthur let Evan go and gave him some grain to take home to his family. His mercy earned Camelot its water back.

However, Arthur failed the second test when he came across the same peasant in the forest. Evan revealed himself to be a thief and a liar and taunted Arthur until he was angry enough to kill him. As he delivered a fatal blow, Evan vanished and was replaced by Anhora, who told Arthur that his willingness to kill a man to defend his pride would cost Camelot dearly. Sure enough, when he and Merlin returned to the castle, they found that the remaining grain supply had rotted, leading Uther to stop distributing food to the people.

Merlin later sought out Anhora and begged him to give Arthur another chance. The sorcerer agreed and sent Arthur to the Labyrinth of Gedref, where he was granted one final test. Arthur and Merlin were set before two goblets, one containing water and the other a deadly poison. All the liquid had to be drunk, but they could each only drink from one goblet. At Merlin's suggestion, Arthur poured the contents of one goblet into the other, then (ignoring Merlin's objections) drank all the liquid himself.

Fortunately, Anhora revealed that it wasn't poison the goblet contained, but rather a harmless sleeping draught. His willingness to sacrifice his life for Merlin proved that Arthur was pure of heart and he passed the final test, lifting the curse from Camelot.

When Arthur and Merlin returned to the castle, they learned that the kingdom's crops had been restored and the townsfolk were bringing in the harvest. Later, in the forest, they laid the Unicorn's horn to rest, after which they discovered that the creature had been resurrected (The Labyrinth of Gedref).

Trivia

 * According to Anhora, though he oversees the tests needed to lift the curse, he does not have the power to cast or undo it (The Labyrinth of Gedref).
 * The curse may be related to elemental magic, as it primarily affects water and plants (The Labyrinth of Gedref).