The final test that Santiago undergoes in this story is to transform himself into the wind. This is something that he is only able to do with the help of the desert, the sun and the wind, and this section of the book charts the conversations that he has with each of these entities. This is a vital part of the process of transformation that Santiago goes through as he purifies himself in order to achieve his Personal Legend. Santiago has to learn to communicate with nature in the common language of the world. This helps Santiago to realise what the alchemist repeatedly tells him, that all of nature is part of the same essence, however small or big, and if he is able to connect with a grain of sand he is connecting with God himself. Note how his transformation into the wind is described:
The boy reached through to the Soul of the World, and saw that it was a part of the Soul of God. And he saw that the Soul of God was his own soul. And that he, a boy, could perform miracles.
This test of transforming himself into the wind is therefore so important because Santiago needs to learn the pantheistic message that dominates this book, and how he is a part of nature, a part of God, and therefore able to accomplish miracles. Santiago is only able to achieve his Personal Legend through learning this vital lesson.