A black medicinal plaster that Ning Yuchan successfully boiled using a copper pot, medicinal ingredients, and sesame oil. The process involved heating until herbs turned dark brown, straining the residue, and continuing to boil until water drops formed beads. The plaster effectively improved Father Huo's leg injury, though it caused side effects including unbearable itching, redness, swelling, and signs of ulceration on the ankle.
