Guan Yu's great sword is there,

At the citadel of Shanhaiguan.

The Green Dragon Crescent Blade was the weapon used by Guan Yu. The Green Dragon Crescent Blade weighed eighty-two pounds and was also known as the Cold Yan Saw, which was used by Guan Yu to behead a number of military generals, and so it was also known in later times as the Green Dragon Crescent Blade.

After Guan Yu was killed, the Green Dragon Crescent Blade was seized by Pan Zhang, a general of Eastern Wu. Later, Guan Yu's son, Guan Xing, killed Pan Zhang to avenge his father's death and reclaimed the Green Dragon Crescent Blade.

Today, the 1,700-year-old Green Dragon Crescent Blade, with its blade pointing eastward, is stored at the Shanhaiguan Castle, where it has become a treasure of the town.

Expanded:

The origin of the Green Dragon Crescent Blade:

It is mentioned in the Dictionary of the Romance of the Three Kingdoms compiled by Mr. Shen Bojun that when Guan Yu followed Liu Bei to start his cavalry in Zhuo County, he went to the local craftsman, Tang Blacksmith, in order to get a weapon as he wanted.

The Tang blacksmith, a local expert in the manufacture of weapons.

The blacksmith made several large swords weighing eighty-two pounds, but they were all unqualified after a trial. Not only was the blacksmith sweating, but Guan Yu was also complaining.

In a moment of desperation, the blacksmith set up an incense burner and swore an oath to God. After three days and three nights of forging, another great sword was about to come out.

Just then, suddenly the sky turned dark and a green dragon fell into the furnace. The blacksmith and Guan Yu were shocked, and rushed to take the sword out to see that the dragon was still attached to the sword.

Guan Yu took the knife in his hand and danced a few times, a cold light flashed in front of his eyes and a cold wind swished in his ears. He raised his sword and slashed at an iron stake on the side, which turned into two pieces like tofu. The Green Dragon Crescent Moon Blade was finally refined.

Baidu Encyclopedia - Blue Dragon Crescent Blade