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 in defiance of the brave soldiers who ineffectually tried to stop him. He reached Princess Zelda herself, whom he presented with an ultimatum: to surrender or die.

Zelda, not wishing to risk the lives of her people against seemingly impossible odds, dropped her rapier sword as a sign of surrender. Twilight surged from Hyrule Castle and covered the land like a shroud, causing much of the Hylian populace to be reduced to spirit form, 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.