Fused Shadow

The Fused Shadow is a quest item from The Legend of Zelda: Twilight Princess. It is the physical manifestation of extremely powerful Shadow magic created by an "ancient race" and once used by a group known only as the Interlopers. This power was sealed away by the Light Spirits.

While the item must be made whole again in order to unleash its full power, just the Fused Shadow shards appear to hold a significant amount of power, as they can curse individuals and transform them into monsters.

History
Before the game begins, the item has already been broken into four shards which, at Midna's behest, Link must find in order to defeat Zant. One Fused Shadow shard is worn by Midna, atop her head as a helmet, while the three others are found in various dungeons in Hyrule, at the Forest Temple, the Lakebed Temple, and the Goron Mines, where they were guarded by the monsters Diababa, Morpheel, and Fyrus. When completed, the Fused Shadow resembles a large helmet.

Soon after the final piece is recovered, it is stolen by Zant and broken, at Lanayru Spring, forcing Link and Midna to pursue him into the Twilight Realm, where they first learn of the behind-the-scenes manipulation by Ganondorf. Using just a fraction of its power, Midna is able to utterly obliterate Zant, and by using more (possibly all) of its power, transform into a spider-like creature capable of piercing Ganondorf's barriers. She succeeds in breaking Ganondorf's hold on Puppet Zelda and transports her and Link out of the castle for her showdown with Ganondorf. However, Midna learns that the power of the Goddesses is far greater than the Fused Shadow, as she is soundly defeated after challenging Ganondorf. Afterwards, he emerges victorious, holding up the shard Midna had worn throughout the game and shattering it.

While it is unclear what happens to the remaining Fused Shadow shards afterwards, as they are shown scattered and half-buried in Hyrule Field at the end of the game's credits and final cinematics.

Sheikah Emblem
At the back of the completed Fused Shadow is a symbol, very similar to the iconic eye emblem of the Sheikah race. The eye part can be seen beforehand on Midna's section of the Fused Shadow, but it is usually hidden by her ponytail. The appearance of the symbol here, as well as in many Twilight-related places, may suggest that the Sheikah were the original creators of the Fused Shadow. Also the Sheikah eye is seen on the throne in the throne room of Zant in the Twilight Realm.

Majora's Mask
One of the Fused Shadow's "eyes" bears an uncanny resemblance to the eyes of Majora's Mask from The Legend of Zelda: Majora's Mask. It is possible that it is somehow connected to the mystical tribe that created Majora's Mask, or to the mask itself as a reincarnation of its power. The creature the Fused Shadow transforms Midna into also resembles Majora's first and second forms slightly (a spider-like being with tendrils sprouting from it and the respective mask as the head).

Animal Crossing: City Folk
Animal Crossing: City Folk features a helmet called "Midna's Mask," which is the Fused Shadow helmet that Midna wears throughout Twilight Princess, along with her hair. It is obtained through the Tom Nook point system for 6,500 points.