A guaranteed Yiga Mask can be found in False God Passage, which is accessed via the Rift Secret Grotto. It is buried in the fireplace just outside of the Cult Headquarters.
The Yiga Mask is a primary piece of the uniform of the elusive Yiga Clan. The masks have become increasingly more common as relics of Hyrule have emerged and the Yiga have become more active.
The origins of the Yiga Mask in False God Passage are unknown. It may have been looted by a member of the Cult of Ganon and later thrown in the fire, or it may have been hidden by a member of the Yiga Clan infiltrating the cult.
Background and Inspiration Edit
The Yiga Mask was added in version 6.5 as a secret item. When secret Yiga enemies were added in later updates, the Yiga Mask became slightly more common.
JKalenad had wanted to implement the Yiga since the release of Breath of the Wild and worked out a context for them in Relics of Hyrule. He plans to expand the Yiga's presence in the mod in each update to the mod, culminating in version 6.7, which will be heavily focused on Breath of the Wild content.