Fai's tattoo is shaped like a pheonix. It had been placed on Fai by King Ashura and was designed to suppress the growth of Fai's powers. King Ashura did this so that his own power would someday grow beyond Fai's, thus activating and consequently breaking Fai's first curse. However, King Ashura erased the memory of his explanation from Fai's mind and for most of his life, Fai believed that the tattoo was meant to suppress the first curse.
The tattoo was given up to Yūko Ichihara in payment for the ability to travel between worlds along with Syaoran, Sakura, and Kurogane. Fai vowed not to use his magic without the tattoo.