Fused Shadow

The Fused Shadows are quest items featured in. They hold the magical power used by the Dark Interlopers in the Interloper War, who sought entry to the Sacred Realm to claim the Triforce.

Locations and Quest Purpose
The enormous power of the Interlopers was sealed away within the Fused Shadow by three of the four Light Spirits, under the order of the goddesses. 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, of the Forest Temple, Goron Mines, Lakebed Temple, and Palace of Twilight, respectively. 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. After she holds out her hand, the Fused Shadow appears. This could be because she, as the "true" heir to the throne of the twilight realm, is entitled to such power, and thus it is likely that she had it with her throughout her life.

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.

Theory
It is speculated by some fans that the Fused Shadow is somehow related to the tribal hexing mask Majora's Mask. The evil mask and the Fused Shadow are somewhat similar in appearance, and both also share dark magical abilities and the potential to corrupt those who come into contact with them (e.g. a fragment of the Fused Shadow turned Darbus into Fyrus, while Majora's Mask forced the mischievous but otherwise peaceful Skull Kid to perform sadistic acts). Given these facts, as well as the fact that Majora's Mask was used by an "ancient tribe", some Zelda fans believe that the Twili (or more likely the Dark Interlopers from whom they were descended, or the Terminan counterparts of the Interlopers) may have also created Majora's Mask.