"We are all driven by two opposite aspects that guide our every action. Regardless of their intention, every decision is the combination of two interconnected yet contradictory aspects. For example, destruction is your only chance of preserving your ideals. While others can achieve peacefulness through nonviolent and pacifistic actions, your existence dictates that you can only achieve peace after destroying your enemies. It is for this reason that you're the only one capable of taking the swords of Yin and Yang to their utmost potential."-Master Huashan.
Hēibái Wúcháng (黑白无常) are two metal swords crafted by the mysterious blacksmith and sword master, Huashan. Passed down his lineage, they were eventually given to Wei Meng as a parting gift before he left Omashu. Although he learned the art of dual wielding swords from the infamous warriors of Kyoshi, only when wielding Hēibái Wuchang did he fully realize its potential. Due to the swords metallic composition, Wei Meng is able to manpiulate the blades masterfully without physical contact through his impressive skill in metalbending. Combining its curviture with his metalbending allows him to switch from various distances by attacking at different angles.
The two swords are said to represent the connection between two opposite aspects that are interconnected, creating the physical symbolism of Yin and Yang. Heibai Wuchang undergo Taiji to actualizing Yin and Yang into the physical realm. The Taiji, a cosmological term for the Supreme Ultimate, is an ancient philosophy that originates from the Shaoshi mountains. The concept of Taiji explores the transformation cosmic singularities, known as Wuji, undergo to become Yin and Yang, undergo. While Yang symbolizes generated activity through the color white, Yin defines stillness with the color black. And within each color, is a bit of the opposite aspect within itself. Yin and Yang simultaneously define contrary forces found throughout nature that, as previously mentioned, share a relationship. Such manifestations include day, night, fire, water, darkness, light, male and female. Physical and conceptual existences are given a complementary and interconnected aspect that is also it's antithesis. Often mistaken as complete opposites, these contrary ideals ultimately give rise to one another. They are the source of the previous existence; as night creates day and day turns into night. Although everything possesses an inverse, it is the fusion of both individual parts that creates an entirely new yet stronger existence. The combination of two opposites into one creates a weapon capable of destroying all before it while causing irreversible damage. Legend has it that when a master is chosen blades, the swords actualize the Yin and Yang aspects that define them. Or put simply, the two opposite forces that dictate the actions the master are predetermined to take. The most recent example being Wei Meng, his murderous entertainment constantly battles against his will and desire to save or preserve those he love. But when utilized together, turn Wei Meng into a killer willing to dispose of those he perceive as evil which, in turn, leads to a prosperous outcome.
Their representation of the Taiji is solidified in the discipline of Nineteen Styles of Jade Maiden; where one blade moves, the other follows directly afterwards. Master Huashan describes it as the motion which leads to stillness which leads to motion which leads to stillness. The day that becomes night that becomes day that becomes night. For when the master parries with one blade, they strike the sibling sword.