Gates of Souls are objects in Hyrule Warriors. They are gateways to other time periods of Hyrule's history, and can be used to summon various things from these other times. There is one located in the Valley of Seers that Cia uses to summon monsters from three different eras of Hyrule. It is also Lana's Level 3 Summoning Gate weapon.
War Across the Ages
Corrupted and manipulated by the imprisoned Spirit of Evil, Cia opens the Gate of Souls to summon hordes of monsters to create her army, the Dark Forces. Using the Gate of Souls in the Valley of Seers, she summoned countless monsters to invade Hyrule, allowing her to essentially overwhelm the Hyrulean Forces with sheer numbers. After joining with the Hyrulean Forces under Impa, Sheik, and Link, the White Sorceress, Lana informed them of the existence of the Gate of Souls. The Hyrulean Forces decided to put their search for missing Princess Zelda in order to close the Gate of Souls and defeat the Dark Forces. In The Sorceress of the Valley, after Link manages to reach the temple summit keep containing the Gate of Souls, Cia uses the Triforce of Power to summon Darknuts, Bokoblins, and a Manhandla from the gate. The Hyrulean Forces defeat Manhandla and Link, Sheik, Impa, and Lana all confront Cia at the summit once more. However Link accidentally triggers a Magic Circle that traps them and allows Cia to steal the Triforce of Courage from Link and Triforce of Wisdom from Sheik. Through Cia the spirit of evil uses the complete Triforce to bend time and space, opening Gates of Souls in 3 Eras where 3 of its Spirit Fragments had been sealed causing various timelines to merge with present day Hyrule. The Hyrulean Forces separate into three groups to close one of the gates in each era.
Era of the Hero of Time
In the Era of the Hero of Time, a Gate of Souls opens up in the Hall of Mirrors keep inside Water Temple. Cia sends Wizzro to guard it using the disguise of Princess Zelda to manipulate the Gorons and demoralize the Hyrulean Forces. In the Land of Myth, Impa and Sheik free the Gorons from the Dark Forces' control and together with sages Darunia and Princess Ruto they invade the Water Temple to close the gate and defeat the imposter. Sheik dispels Wizzro's disguise with the Lens of Truth and Sheik reveals herself to be the real Princess Zelda. After Wizzro flees, Zelda and Impa close the gate together.
Era of Twilight
In the Era of Twilight, a Gate of Souls opens in the throne room of the Palace of Twilight located in the Twilight Realm. Cia invades and manages to force the Usurper King of Twilight, Zant into allying with the Dark Forces. Cia turns Midna into an imp and sets up a small base in Twilight Field. In the Land of Twilight, Lana and her troops have trouble finding the gate's location due patches of Twilight covering Twilight Field. Fortunately, Lana saves a young lady named Agitha who shows her a Goddess Butterfly that can lead her to where she wants to go. However Midna ends up capturing the butterfly and uses it to track down Cia, who promptly flees. Mistakenly believing Lana and her group are enemies, an angry Midna and her forces attack them in retaliation for Cia's escape, forcing Lana to defeat her. After explaining they are not enemies, Midna joins Lana's group and using the Goddess Butterfly they discover the Gate is located in the Twilight Realm. They invade the Palace of Twilight, the are confronted by Zant, who Cia had left to guard the Gate of Souls. After defeating Argorok and its master Zant, Lana closes the Gate of Souls. However Cia appears to gloat and mock Midna who remains under her curse, causing Midna to sucker punch her with her hair, knocking off Cia's mask in the process. As Cia flees, Midna is stunned by Cia's uncanny resemblance to Lana, forcing Lana to reveal that she and Cia where once the same person.
Era of Skyloft
In the Era of Skyloft, Cia's use of the Triforce causes the Gate of Time to be corrupted and transformed into a makeshift Gate of Souls.
In Linkle's Tale: The Demon Lord's Plan, Ghirahim takes advantage of the monsters coming through the gate in order to awaken his master. However his ritual to revive his master fails and instead releases several miniature versions of The Imprisoned, forcing Ghirahim to use the Gate of Souls to increase the ritual's power. However Fi and Linkle's Forces interfere with Ghirahim's plans.
Ghirahim would later lead his forces in an attack on Skyloft and even manages to summon The Imprisoned within Skyloft. Cia manages to convince Ghirahim to ally himself with the Dark Forces and sends Volga to support him. In the Land of the Sky, Link and Proxi arrive in Skyloft to find the Knights of Skyloft fighting a losing battle with the Dark Forces lead by Volga and Ghirahim. Fortunately, Link's presence awakens Fi, the spirit of the Goddess Sword and she offers to assist him. Link manages to drive off Ghirahim, who leaves the battle to Volga. Calling upon the power of the Sky Spirit Levias they manage to weaken and defeat Volga. However Ghirahim decides to use his power to corrupt the minds of both Skyloft and Hyrulean soldiers. In Sealed Ambition, Ghirahim unleashes his army of brainwashed Turncoat Soldiers to attack and demoralize Link and Fi's allied forces, forcing them to track down the Turncoat Leader. After his plan to destroy his enemies from within fails, Ghirahim uses the gate to summon his master, The Imprisoned. Using the Groosenator, Link and Fi manages to weaken The Imprisoned and eventually defeat it, causing a furious Ghirahim to open the doors of the Sealed Temple allowing Link & Fi to enter the Temple and defeat Ghirahim in the Gate of Time's chamber. Link manages to knock, the defeated Ghirahim into the Gate of Souls and then Fi instructs him to raise his sword skyward to fire a Skyward Strike that effectively restores the Gate of Time, closing the Gate of Souls.
In Shining Beacon during the final battle between the Dark and Hyrulean Forces, Cia having become fully corrupted by darkness uses the Gate of Souls in the Valley of Seers to continue summoning monsters despite Lana's pleas to stop, as doing so was taking a toll on her soul. She even ends up creating a second Gate of Souls, forcing the Hyrulean Forces to take Cia down in order to defeat the Dark Forces. Following her defeat, Cia collapses and dies in Lana's arms due to the damage done to her soul. Lana takes possession of the Triforce of Power and uses her power to create three golden-colored Gate of Souls to send their allies (Darunia, Ruto, Agitha, Midna, and Fi) back to their respective Eras and closes the Gate of Souls in the Valley of Seers.
Despite the Dark Forces defeat the spirit of evil, Ganondorf escape from his imprisonment and manages to obtain the complete Triforce by claiming the three pieces from Lana, Link, and Zelda. Confident in his victory, Ganondorf disappears from the battlefield and uses the Triforce to summon monsters to bolster his forces as Cia had done with the Gate of Souls. Refusing to give up, the Hyrulean Forces invaded Gerudo Desert to take down Ganondorf's generals, the revived Ghirahim and Zant. In Enduring Resolve, Lana uses a Magic Circle to create the three golden Gate of Souls she had used after Cia's defeat to summon their allies from across the ages, who quickly turn the battle around in the Hyrulean Forces' favor, resulting in Ghirahim and Zant's defeat. Following Ganon's defeat, it is implied that the power of the Triforce returned the Hyrulean Forces allies back to their respective Eras without needing to use the gates Lana used to summon them, as they (Darunia, Ruto, Agitha, Midna, and Fi) simply fade away shortly after Ganon's defeat.
Wind Waker: The Search for Cia
Though it seemed that Hyrule had seen the last of the Gate of Souls with the defeat of Cia and Ganon, it was not the last Hyrule would see of the Gate of Souls. Lana returned to the Valley of the Seers to search for Cia, who had mysteriously vanished following her defeat. However while Lana was busy searching via her crystal ball, an mysterious dark figure attacked her and stole the Triforce of Power. The mysterious figure used the Triforce of Power to warp time and space, causing the Era of the Great Sea to appear in Hyrule. Believing that these events were connected to Cia's disappearance, Lana journeyed to Gerudo Desert where several Gates had been opened causing monsters to come through them once more, defended by Big Blin of the Monster Forces. Lana managed to close two of the gates by defeating the Big Blin guarding them, when the Great Bird Helmaroc King appeared in the Arbiter's Grounds with its captive Tetra. At first, Lana chose to ignore the bird, believing it was only looking for a place to roost. However as the bird rested, Tetra snuck away to the southern part of the desert, insisting that Lana and her allies provide her some assistance. Soon Link and Proxi arrived and revealed that Tetra was an ally of theirs. Unfortunately, Helmaroc King woke up and attempted to retrieve Tetra. Together Link and Lana managed to drive off the bird and rescue Tetra. Eventually they managed to defeat all the Big Blin and close all the gates, before turning their attention to finishing of Helmaroc King. Following the Great Bird's demise, they encounter a legendary figure from the Era of the Great Sea. It was none other than the Daphnes Nohansen Hyrule, the King of Red Lions himself. King Daphnes explained that the mysterious figure that stole the Triforce of Power had originated from the Era of the Great Sea. Joined by King Daphnes, Lana, Tetra, and Link journeyed to the Temple of Souls to find Cia and the villain responsible for the warping of time and space.
Cia and Lana each use their own version of the Gate of Souls.
Gate of Souls (Light Version)
Lana's version of the Gate of Souls is a gold Summoning Gate that she uses as her Level 3 weapon in her Summoning Gate moveset, allowing her to summon Cuccos, Manhandla Stalks, and miniature versions of King Dodongo, Gohma, and Argorok. In the main story, Lana can use a Magic Circle to create three gates to transport her allies to and from their respective Eras. She can also close gates created by Cia by herself (as Zelda & Impa had to combine their magic and Link required the power of a Skyward Strike in order to do so).
Gate of Souls (Dark Version)
Cia's version of the Gate of Souls is basically a corrupted version of Lana's that Cia uses to summon monsters to serve in the Dark Forces. Via the use of the Gate of Souls, Cia can summon ever greater numbers of monsters, allowing her to overwhelm her enemies via sheer numbers, though doing repeatedly is dangerous and taxing. Her ability to use them is further enhanced by her possession of the Triforce, which allows her to warp time and space, creating a Gate of Soul in each era. Outside of the story, the top of Cia's Scepter of Souls features her version of the Gate of Souls as part of its design and appears in the background of certain stages.
Gate of Time (Corrupted)
In the Era of Skyloft, Cia's misuse of the Triforce causes the Gate of Time inside the Sealed Temple to be converted into a makeshift Gate of Souls. It is later restored by Link's use of the Skyward Strike.