It's not obvious where to find it: look in the forest in northern part of Ninjinia. Other than that it's relatively simple.
If you are playing kana mode you will need 700 Kanji to navigate.
There are several chests, some of which contain herbs, and one of which contains the quest item: Odaicon's scythe. Odaicon's scythe can be used on the walls of the maze to obtain various types of herbs.