Why is Zoro new sword called Enma

Zoro new sword its called Enma because of way he get it.

Zoro frist gets Shusui  one of the 21 Great Grade swords that was once wielded by the legendary samurai Ryuma. It is prized as the national treasure of the Wano Country.

 After Hiyori begged Roronoa Zoro to leave his stolen Shusui, an O Wazamono grade sword in its rightful place as a national treasure in Wano though he earned the sword from its previous zombified owner, she offered Enma as an equal replacement. Zoro agreed to Hiyori's proposal to return Shusui for Enma.

In real life  ENMA is a non-profit organization staffed by a team of community minded volunteers who are interested in preserving and furthering the arts and crafts of Japan for the enjoyment of all community members in the Greater Seattle area and Pacific Northwest.

 Kozuki Hiyori  trade Enma with Zoro for  Shusui.

 Even thought Enma is more powerful than Shusui it cant be controlled

For people in Wano Shusui have more value than Enma .It holds more value because :

   1.  Shusui   was hold by Ryuma and it was his only sword it was always used

     While  Enma was on of two swords wielded by Kozuki Oden. In fights Oden  didn't always

 use his swords.

2. After Ryuma death Wano was at peace. 

 Ryuma was bestowed the title of "Sword God", with his corpse still retaining its status as one of Wano's national treasures. 

The tale of his fight against the dragon became legendary and was passed down the generations.

 After Odin death Wano not peacefully

His death was deeply mourned by the rest of the country and his noble reputation was fully restored to the rest of the people of Wano, causing tons of people to take arms and ready to rebel against Orochi's regime and the Beasts Pirates in his name and the Kozuki Family. 

Sympathizing with the Kozuki Family became a high crime, one that even children were punished heavily for committing.

     

So i think that Zoro new sword is called Enma because  Kozuki Hiyori did same thing as that 

organization do she traded  official  national treasure (Shusui) for non-official national treasure (Enma ) 

 Enma is also   the god of hell in the Buddhist religion. He is often called Yama. He is the god who judges whether the souls of people go to heaven or hell.

Zoro's new ownership of Enma is in accord with his character theme of Buddhist spirituality. 

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