The horseman in Paulo Coelho's The Alchemist is actually the alchemist that Santiago has been searching for. However, when the boy (Santiago) first sees the rider come pounding up to him, he has no idea of the man's real identity.
The first thing the horseman demands to know is the identity of the person who has dared to read the omen of the fighting hawks.
Suddenly he heard a thundering sound, and he was thrown to the ground by a wind such as he had never known...Before him was an enormous white horse, rearing over him with a frightening scream...
(The presence of the wind here is foreshadowing.) Santiago gets his first glimpse of the horseman who has all but trampled him with his animal. The man is dressed completely in black, with a falcon resting on his arm. Drawing his sword, the man speaks.
"Who dares to read the meaning of the flight of the hawks?" he demanded so loudly that his words seemed to echo...
Santiago does not hesitate, but identifies himself as one who has "dared to do so." The boy is prepared to die by this sword if he has done something to offend, but calmly explains that he has saved lives by doing so. Rather than killing Santiago, the horseman asks why he read the omen of the birds. Santiago answers that he "read" what the birds wanted him to know, which was that the oasis was in danger—in this way, he could save the people there. The man in black then asks:
Who are you to change what Allah has willed?
The boy explains that Allah has created all things, as well as the birds who gave him the message. The horseman lowers his sword but warns Santiago:
Be careful with your prognostications...When something is written, there is no way to change it.
Santiago says that he had a vision, and then tells the man that he is following his Personal Legend, thinking that this man could not possibly understand what that is. The man does know (and so much more that Santiago could never imagine at that time), and points out that he had to see how brave Santiago was—which is why he seemed so threatening—for bravery is absolutely necessary if the boy is to understand the "Language of the World."
The man's final advice to Santiago is that he must not give up on his quest. Then he warns him:
You must love the desert, but never trust it completely. Because the desert tests all men: it challenges every step, and kills those who become distracted.
Santiago now knows he must be more vigilant than ever. The man in black tells Santiago that "if your head is still on your shoulders at sunset," the boy should find him. The young man asks where the horseman lives, and the direction in which the man points lets Santiago know he has finally met the alchemist.