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 four 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.

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 Ganon. However, the power of the Fused Shadows pales in comparison to the power of the Goddesses, as seen when Midna challenges Ganon, and is soundly defeated. Ganon emerges from the encounter unscathed, grasping the fragment of the Fused Shadow that Midna wore as a helmet. He then 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.