The Kanohi Dexter is the Mask of Reversion. When used on a state of matter, said matter would be reverted to its previous form. For example, if a pack of ice cubes had melted into water, the Dexter could be used to turn them back into ice cubes again, without leaving any trace of their transformation. Theoretically, it could also be used to turn Toa into Matoran, butterflies into cocoons... you get the idea.
Only one was known to exist, which belonged to Shattered Mirror Kakamu, but it was destroyed along with his original body. It is unknown if any exist in the BZPB Multiverse, or even if they can be made with the main multiverse's Kanoka disks.