The Sealed Grounds, formerly known as Hylia's Realm, is a location from The Legend of Zelda: Skyward Sword.
Before the events of the game, the Sealed Grounds served as a place of worship for the goddess Hylia. It is here that Demise was sealed away in the form of The Imprisoned. The Sealed Temple also stands on the Sealed Grounds. This temple is where Link lays the Master Sword to rest at the end of his adventure.
The artwork from the game's prologue can be found along the bottom walls of the grounds, depicting the attacking demons, Skyloft's creation, and the races who remained on the surface with the goddess to imprison the demons.
During the events of the game, Link visits the Sealed Grounds numerous times. It is the first area of the surface to which Link descends. At the bottom of the large pit in the Sealed Grounds lies the sealing spike which keeps The Imprisoned incarcerated. Link must restore power to the seal using a Skyward Strike when he first arrives at the grounds. Link returns after obtaining the Goddess's Harp from Zelda, and is followed by Groose, who pursued Link in the hopes of being the one to find Zelda. Link learns the Ballad of the Goddess from the Old Woman in the Sealed Temple, awakening a Gate of Time as he plays it. However, the awakening of the gate stirs The Imprisoned, which breaks free of its seal. Link is able to defeat the creature and restore the seal temporarily. At this point Groose realizes that Link is truly the hero meant to find Zelda, and remains in the Sealed Grounds. The Old Woman convinces him that he has an important role to play, and Groose goes on to build the Groosenator to help battle The Imprisoned in the future.
After tempering his sword into the Master Sword, Link returns to the Sealed Grounds to activate the Gate of Time, but the Imprisoned once again escapes its seal. Link, with the help of Groose and his Groosenator, manages to seal the Imprisoned once again.
Later on Link returns to learn part of the Song of the Hero from Faron, but is forced to land in the Sealed Grounds as the Faron Woods have been flooded. The Imprisoned escapes for a third time, but Link again reseals it with Groose's help. Groose then catapults Link into the flooded Faron Woods to continue his quest.
The Sealed Grounds, as well as the Sealed Temple, is one of the playable stages in the spin-off game Hyrule Warriors. Much like in Skyward Sword the sealing spike is present at the center of the grounds, although instead of one Groosenator on rails, three such devices are located in three keep areas.
In the main story the Temple housed a chest that locked away one of the fragments of Ganondorf's soul. The Dark Sorceress Cia opened a Gate of Souls inside the Temple that released the chest's bindings and freed the spirit fragment, while also allowing swarms of monsters to overrun the Skyloft realm. After rescuing Skyloft, Link and Fi pursue Ghirahim to the Sealed Grounds to close the Gate of Souls there, but are forced to battle their own soldiers that turned traitor as well as defeat the Imprisoned that Ghirahim released. Once Ghirahim is defeated, Link performs a Skyward Strike to close the Gate of Souls.
An alternate version of the Sealed Grounds stage appears in Linkle's Tale: The Demon Lord's Plan story scenario, which features a sky covered by dark clouds. In this scenario, Ghirahim and his Ceremonial Troops attempt to revive The Imprisoned in response to the Gate of Time being transformed into a Gate of Souls by Cia's misuse of the Triforce to bend time and space. Sensing a disturbance in her timeline, Fi journeys to the Sealed Grounds in order to stop Ghirahim, aided by Linkle and her Cuccos.
It is possible the Sealed Temple later became the Temple of Time seen in The Legend of Zelda: Ocarina of Time. This is due to to structural similarities, such as a sword pedestal in the back, large doors and tall ceiling. There is also the war of time that means it could become the Temple of Time. The town center could also be what becomes of the circular area outside, due to the circular nature and proximity to the Temple of Time. Additionally, the Temple of Time from The Legend of Zelda: Twilight Princess is located in Faron Province, which corresponds to the Sealed Grounds' location. Though Hyrule Historia does not confirm this theory directly, it does state that this is one possibility.
It is possible that the seal holding back Demise has a relationship to the Fused Shadow from The Legend of Zelda: Twilight Princess. Both are ancient artifacts made of a dark substance that bears considerable magical power, and are both capable of transforming those that come in contact with into massive, twisted versions of themselves. Additionally, the process of the seal absorbing the Imprisoned greatly resembles the appearance of the Fused Shadows reconstituting themselves, as the massive monster in question explodes into many black fragments, which are then pulled into a central point.