Fused Shadow

The Fused Shadows are items that Link must acquire for Midna in return for her help. There are three Fused Shadows that are to be collected during the game and are held by the dungeon bosses Diababa, Fyrus, Morpheel, and later Zant. However, there are four in all - Midna's helm is the fourth Fused Shadow.

The pieces of Fused Shadows hold the magical power used by the Dark Interlopers who wanted to invade the Sacred Realm to get a hold on the Triforce and its powers. The power was sealed within these objects and locked away. Anything that comes too close to this dark power is mutated, as the bosses of each dungeon reveal. This is especially apparent in the Goron Mines, where the Patriarch of the Gorons was transformed into Fyrus by the dark power of the Fused Shadows.

 The Fused Shadows were used by Midna to defeat Zant. Their power was also used to break through Ganondorf's barrier around Hyrule Castle and almost managed to defeat Ganon himself, however they were presumably permanently destroyed when Ganondorf smashed Midna's helmet, the fourth piece of the Fused Shadows, after besting her in combat.

It is also proven that the Fused Shadow is related to the tribal hexing mask Majora's Mask. All Zelda fans and obsessors have long debated of how the evil mask and the Fused Shadow are similar, leading to the conclusion that the Twili created the Majora's Mask to take over Hyrule. But the mask's power was too great for them, and so they sealed it away before being imprisoned in the Twilight Realm.