The Kanohi Keemaru is the Great Mask of Confusion.
The Kanohi Keemaru was created in an experiment, led by Makuta Artorius, designed to create a mask that would wipe a target's subconscious mind completely. While the initial concept never came to fruition, the resulting mold was found to be quite powerful. Thus, to prevent others from creating a second Kanohi Keemaru, Artorius stole the prototype and killed everyone on the research team who had helped to create it.
A powerful but rare mask, the Kanohi Keemaru allows its' user to temporarily distort a target's sense of time and place, effectively erasing their short-term memories, and opening them up to the false memory suggestions of the user.