The "Masked Breen" was the name given to the first Breen to visit Ferenginar, sometime between the mid 20th and mid 22nd centuries. He was given this name by the Ferengi whom he encountered due to the spacesuit helmet which he continuously wore. The Ferengi were unaware that no Breen ever removed their helmets in alien atmospheres.
The Masked Breen was responsible for selling warp drive to the Ferengi, for which he charged the price of Ferenginar's North and South Poles, plus seven remote moons and six comets in the Ferenginar star system. The Masked Breen's sales pitch -- "Expand... or die" -- became the 95th Rule of Acquisition. (DS9 reference: Legends of the Ferengi)