Fused Shadow

The pieces of Fused Shadows hold the magical power used by the Dark Interlopers who sought entry to the Sacred Realm to claim the Triforce. The enormous power of the Interlopers was sealed away within the Fused Shadow by the Gods. The Fused Shadow was then shattered, its pieces concealed deep within the temples of Hyrule. Anything that comes in contact with dark power radiating from the Fused Shadow is transformed, as were the bosses of the first three dungeons. 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. Midna entreats Link to recover the Fused Shadows 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.

Though it is unknown exactly how and why Midna obtained the last Fused Shadow there is a short cut scene showing Midna, shortly after being transformed into an imp, walking away from the palace of Twilight when her helm suddenly floats down in front of her (this could be because she, as the "true" heir to the throne of the twilight relm, is entitled to such power and thus, like the triforce pieces, the power chose her as it's wielder).

Powers and Abilities
The true power of the Fused Shadow is immense; using just a fraction of its power, Midna obliterated Zant, and when Midna dons the complete Fused Shadow, it transforms her into an enormously powerful spider-like creature, capable of piercing the otherwise impenetrable barriers created by Ganondorf. However, the power of the Fused Shadows is less than that of the power of the Goddesses, as seen when Midna challenges Ganondorf, and is soundly defeated. Ganondorf emerges from the encounter seemingly unscathed, grasping the fragment of the Fused Shadow that Midna wore as a helmet.

However, this may also be because the Fused shadow's power may have withered over time. This can be assumed based on the damaged and cracked condition of the actual relic, and Zant's reference to it as "ancient and withered." Also, though Ganondorf is unharmed physically, he is no longer in the form he was when he fought Midna; possibly because she weakened him.

Ganondorf crushes the fragment, presumably destroying the power of the Fused Shadow itself. It is unknown what ultimately became of the crushed fragment of the Fused Shadows. In the end credits, its pieces are seen lying half-buried in Hyrule Field.

Theories
It is speculated by some fans that the Fused Shadow is somehow related to the tribal hexing mask Majora's Mask. Considering that the evil mask and the Fused Shadow are somewhat similar in appearance, and given that Majora's Mask was created by "an ancient tribe", some Zelda fans believe that the Twili (or the Twili's Terminan counterparts) may have also created Majora's Mask. The spider-like creature that the Fused Shadow transforms Midna into could be considered similar in appearance to that of the Majora Beast.