A Unicorn is a magical creature that inhabits the lands of Albion, including Camelot. It is a mythological creature thought to be linked with powerful magic. The legends state that if a unicorn is killed then a curse will fall upon its killer. The only way to lift the curse is if the killer proves themselves to be pure of heart, like the unicorn. The unicorn is protected by Anhora, the keeper of the unicorns.
While on a hunting trip, Merlin encountered a unicorn. Although he tried to send it away, Arthur shot the unicorn dead and took its horn as proof of killing it. Gaius warned both Arthur and Uther of the curse associated with the unicorn but neither of them listened. However, within a few days, the crops began to die and the wells drained of water. Arthur was then told by Anhora that the death of the unicorn meant he had to be tested to prove his worth if he wanted the curse to be lifted.
After Arthur passed the final test in which he proved his heart to be as pure as that of a unicorn by willingly giving his life for Merlin's and accepting responsibility, the unicorn's curse was lifted. Arthur and Merlin went into the forest to lay the unicorn's horn to rest, at which point they saw the unicorn restored to life (The Labyrinth of Gedref).
In British mythology, the Unicorn was also a very shy creature. It was highly sought for by hunters and many traps were supposedly designed to capture them. The most common belief around the unicorn was that only a virgin could tame it. Therefore these traps would involve placing the virgin within them and luring the unicorn in.