From the introduction:
The mystic is an entire class to try out in your D&D games. This version of the class arose out of playtest feedback on two previous versions.
[Mystics are] followers of a strange and mysterious form of power. Mystics shun the world to turn their eyes inward, mastering the full potential of their minds and exploring their psyches before turning to face the world. Mystics are incredibly rare, and most prefer to keep the nature of their abilities secret. Using their inner, psychic strength, they can read minds, fade into invisibility, transform their bodies into living iron, and seize control of the physical world and bend it to their will.