The operation here is assassination. What weapons can be used for assassination during the Three Kingdoms period? The first and most likely scenario is that it is a tracker and a thin man, similar to the HongLing gun that appeared in the Han Dynasty, and much longer than a spear or gun. I hope Guan Yu rushes ahead of the enemy and tries to kill Yan Liang. After all, during the Romance of the Three Kingdoms, slimming was a conventional weapon, and Cao Cao introduced slimming into poetry.
The second possibility is spears or guns, both of which are relatively old weapons, but spears were widely used only after the Tang and Song Dynasties, and guns are relatively old primitive weapons. With Guan Yu's military talent, using these two weapons is a bit overwhelming, but it is also possible.
In addition, there are also records of beheading in the annals of the Three Kingdoms. Could it be a sword? I don't think so. Why? It is not easy for Guan Yu to stab Yan Liang to death. However, Guan Yu may carry it with him as a backup weapon. For example, in Romance of the Three Kingdoms, Guan Yu cut off Yan Liang's head with a round arch. So the conclusion is that Guan Yu's real weapon may be a tracker, which is Cao Cao's tracker.
Summary: To sum up, Guan Yu's weapon is a double knife, which can be cut and stabbed. The word "ten thousand people" is engraved on the knife, which is consistent with the "enemy of ten thousand people" mentioned in the History of the Three Kingdoms: Zhang Yide and Guan Yunchang are enemies of ten thousand people. In ancient romance novels, "a man is brave enough to be a man" is what it means. Moreover, this knife was also used to assassinate Yan Liang after the war. Later, he defeated Maicheng. In order to prevent the double knives from falling into the enemy's hands, he threw himself into the river. Later literati daydreamed about Guan Yu's courage, and ten thousand knives became a dragon crescent moon knife, which was famous throughout the ages.