Shadow Invasion

The Shadow Invasion is the name of a large-scale invasion launched against the land of Hyrule by Zant, the usurper King of Twilight featured in Twilight Princess.

Zant's Rule
Over a century after the Twili's exile from Hyrule, the Twilight Realm entered a period of political instability when an evil Twili named Zant usurped the throne from the true heir, Princess Midna. Zant had been a servant of the Royal Family of the Twilight Realm, and was embittered that he was not chosen as successor. With the ascension of Midna to the title of Twilight Princess, Zant became emotionally unstable and severely depressed. It was in this state that he was approached by the spirit of Ganondorf, who posed as a god and offered an exchange: he would give Zant the power to take the throne by force, and once he was secure as the ruler of the Twilight Realm, Zant would release Ganondorf back into Hyrule.

Zant transformed the Twili into Shadow Beasts and attacked the Palace of Twilight, where he overthrew Midna by force and transformed her into an impish creature before she escaped Zant's coup. He then sealed away the Sols, the source of light in the Twilight Realm, within the Palace of Twilight, leaving the land in darkness.

The Invasion of Twilight
Zant opened the gateway between the two worlds and used his sorcerous powers to shroud the areas he conquered in twilight. Zant first targeted three of the four Light Spirits that had been responsible for locking the Dark Interlopers away in the Twilight Realm long ago. He sent his Shadow Beasts to steal their light and divide it up amongst a multitude of twilit insects that would scatter around the provinces. The Twili army conquered province after province with ease until they reached Hyrule Castle.

The Twili army eventually even reached the gates of the fortified Hyrule Castle Town itself. He marched into the throne room with Shadow Beasts following him. The soldiers who tried to defend the room were easily subdued. Princess Zelda raises her rapier sword as a final form of defense, but Zant threatened to kill all of her subjects if she did not comply with his order to abdicate the throne to him.

Zelda, not wishing to risk the lives of her people against seemingly impossible odds, dropped her rapier sword as a sign of surrender. Hyrule Castle is set ablaze during this assault. Then, Twilight surged from Hyrule Castle, turning the watching residents of Hyrule Castle Town and the majority of Hyrule's population into hopeless light spirits, unaware of what had happened to them. With this, the land was under Zant and his master Ganondorf's control.

Consequences
With the exception of Ordona, who is saved by Link when a Shadow Beast is coming to steal her light, the Spirits of Light lose their power when their light is seized by the Shadow Beasts. This brings direct effects that, in one or another way, affect the environment of the local provinces. In Faron Woods, a purple toxic fog impedes direct access to the Forest Temple and, even after the province recovers from twilight, remains as a permanent effect. In Death Mountain, the surrounding volcano shows a fiercer-than usual activity, which results in the fall of tectonic material into the Gorons' whereabouts, endangering them. Zora's Domain, as well as surrounding zones like Zora's River and Zora's Waterfall, are frozen; this in turn results into the Zora population remaining trapped inside the ice, and Lake Hylia lowering its water level drastically.

Aftermath
After Link is told to find a way to restore light to Hyrule, he explores the affected provinces, one by one, in order to retrieve the Tears of Light and give the Spirits the ability to expunge the influence of twilight in the regions. Every time the young hero enters a darkened region, he's turned into Wolf Link, but recovers his Hylian form when light comes back in said region. After managing to retrieve the pieces of the Mirror of Twilight, and infiltrating the Palace of Twilight, Link finds Zant, who reveals that a god came down to him, and gave him part of his power. Zant battles Link, by replicating all of the boss rooms in the past dungeons, but Link defeats him. Zant reveals that Midna can't have her true form back, since he does not have that power. In her fury, Midna uses the newly retrieved Fused Shadows to creates large spears, which impales Zant and causes him to explode.

Link goes to Hyrule Castle, and Midna uses the Fused Shadows to smash the twilight pyramid surrounding the castle. Link enters the castle, and finds Ganondorf, and a catatonic Zelda in the throne room. Ganondorf gloats Link and Zelda, and then possesses Zelda, to battle Link. Link wins the battle, and Midna forcefully takes Ganondorf out of Zelda. Ganondorf transforms into a large boar-like creature and battles Link again, but is once again, defeated. Midna opens up a twilight portal and tells Link and Zelda that she will fend of Ganondorf.

After teleporting on Hyrule Field, they think their problems are over, but a large explosion destroys the castle, and Ganondorf, on his horse appears, holding aloft Midna's helmet, which shows that he has defeated her. Link and Zelda fight Ganondorf on Epona, to which Link again wins. Ganondorf then has a sword duel with Link, but drives the Master Sword into Ganondorf's chest. Injured, Ganondorf sees the Triforce of Power disappear from his hand, and sees Zant, whose neck snaps to one side. Ganondorf then presumably, dies. The Light Spirits revive Midna, who regains her true form. Midna then enters the Mirror of Twilight, and with her tear, smashes it, breaking the barrier between the Twilight Realm and Hyrule. The land then gradually redevelops over time.