This medicine originally had no name. Because the Master met an old man in the mountains who claimed to be surnamed Xu, and pointed out this plant to instruct him, it was named "Xu Xianzhi Fang San" (Mist Immortal Rhombic Powder), and everyone henceforth called it by this name.

According to tradition, the round form of Xianzhi belongs to Yang, and the square form belongs to Yin; they are born in opposition as Yin and Yang. The leaf surface is green, and the back is purple-black. The root tuber resembles a centipede in shape, with four leaves growing opposite each other. It flowers in May, has flowers but no fruit, does not wither in winter, and grows in clusters, distinct from other herbs. It grows in deep mountains and large marshes among water and stones. It should be harvested on the fifth day of the fifth lunar month. After harvesting, select the roots to make them uniform, wash away sand and soil by swishing them in rapid running water, and hang them in a place with slight wind and sunlight to dry. Do not bundle them tightly or stack them, lest they rot and become inferior, resulting in weak odor and potency. Store the round and square forms separately in different places; do not mix them. Once dried, when used temporarily, take two roots each of the round and square forms, pack them separately, and ensure no confusion occurs. When using, grind them with a millstone; do not let them touch iron. Grind until extremely fine. Avoid letting monks, nuns, women, mourners, chickens, dogs, and all unclean things see it. Select auspicious days such as Tiande, Yuede, or Tianyi, or the seventh day of the seventh lunar month for compounding. If these rules are followed, the efficacy will be miraculous. Negligence and failure to follow these methods will result in delayed efficacy. Since you revere this medicine, you must be devout.

To prepare this powder for detoxification and resolving sores, mix it with strong vinegar. Apply the root paste to the head of the lesion: apply the round form to the right side and the square form to the left side. The effect should manifest within one shichen (two hours). If severe pain persists, administer another dose. If it still does not subside, add the following prescription:
Xianzhi Genye (Roots and Leaves of *Pseudolarix* / Specific Herb) (1 liang 5 qian total) Zishengsheng (Parasitic Twig of Catalpa) (0.5 liang) Yeming Sha (Guttae Stercoris Cypetini) (1 liang)
Grind the above ingredients according to the previous method, mix thickly, and apply externally. If wind-toxin is extremely severe, add Zhaohouxu (1 liang, known as Baijiangcao in Materia Medica) and Sigua (0.5 liang; fresh is best. In winter, harvest before frost and store; grind into powder for application when needed. Fresh ones should be finely chopped, pounded in a stone mortar, and juiced. One zhan [unit] equals one liang. Do not use vinegar; mix only with juice for better results. This herb also possesses Yin and Yang attributes: long and thin is Yang, short and fat is Yin; they can be used together). For wind-toxin fire-dan resembling boiling liquid splashes, add Yeming Sha (cumulative half liang), Zhen Niuhuang (Real Calculus Bovis, one zi), and Xijiao (Rhinoceros Horn, half qian ground with water, or finely powdered otherwise). Follow the previous method, but do not use vinegar; instead, mix with Sigua juice and water-ground Xijiao for external application.