Snee in Yo-kai Watch 1

Fuse Chansin with a Cursed Blade.

The item can be obtained as a reward from the quest Precious Illusion, given by the man on the Bamboo Path in Blossom Heights, starting from Chapter 11 after unlocking Rank B. After taking the request, defeat Benkei within the Old Mansion to obtain the Illusion Blade and return it to the man, who will let you choose your reward between a Legendary Blade and a Cursed Blade.

After clearing the request, more of the same item can be purchased as Jungle Hunter in exchange for 2 Firefly Squid (Rare) and 3 Locusts (Rare).

Alternatively, a Cursed Blade can be obtained as a reward for clearing the "Snap a Sword Yo-kai" Yo-kai Cam Daily Challenge.