The Fused Shadow is a magical mask found within The Legend of Zelda: Twilight Princess for the Wii and GameCube. In the game, the protagonist Link and his companion Midna sought out the pieces of the mask, specifically for Midna to don.
It was created to seal the evil powers of the Dark Interlopers who wished to enter the Sacred Realm to take hold of the Triforce. Following this, they shattered the mask, and placed each piece within separate dungeons in all of Hyrule. Midna has in possession the fourth piece - which is in fact her helm.
Using the power of the Fused Shadow, Midna was capable of destroying Zant, though when merged she transformed into a horrifying beast that she used against Ganondorf's barriers, thus giving Link access to him. Before Link battles him, however, Midna attempts to do his job that he was entrusted with, and inevitably is defeated by the hands of Ganondorf, who has the power of the Goddesses due to the fact that he wielded the Triforce of Power.
Midna's helm, however, has the same eye as the eyes of Majora's Mask. If you were to place Midna's helm in front of the Majora's Mask, It would be like nothing was there, except for the gray color, expanded parts, etc.