In The Legend of Zelda: Twilight Princess, the Fused Shadows are three items that when combined have some serious power. Link goes to three dungeons to find them.
Once Link fills up the Vessel of Light and returns the Faron Province to light again, The Light Spirit Faron tells him that in order to save Hyrule, he must banish the King of Shadow. However, Link does not have the necessary power to defeat Zant. Faron than explains that there is a power, that was hidden by the Godesses, and whom told only The Light Spirits their locations. According to Faron, the Fused Shadows are the only way to defeat Zant.
The first temple visited by Link is the Forest Temple, where he finds the first piece of Fused Shadow. He than goes to the Eldin Province, where The Light Spirit Eldin tells him that the next Fused Shadows is in Death Mountain. Thus Link goes to Death Montsin and goes into The Goron Mines to receive the second piece of Fused Shadow. Last but not least, The Light Spirit Lanayru tells Link the location of the final Fused Shadow, The Lakebed Temple. Once Link completes the Fused Shadow, he is assaulted by Zant, who than takes the Fused Shadows.