Mirror of Twilight (Total War)

When playing as the Twili
The mirror of Twilight is a special building that can be created in the Palace of Twilight, one already exists in Arbiter's Grounds.

It takes 6 turns to build and costs 2000 rupees.

It allows you to send armies from the Twilight realm into the Gerudo Dessert from where you can begin to invade Greater Hyrule.

In the campaign
During the freeform campaign after a certain requirement is met, an event will trigger and the Forces of Twilight will spawn just outside Arbiter's Grounds and declare war on whichever faction controls that settlement, there armies will then continue to spawn each turn.

History
The mirror is believed to have been created by the Druthulidi Bellum during Hyrule's creation, in order to summon armies from it's previous conquests. However sometime during the Ancient Age Bellum entered a state of slumber and the portal was abandoned, and at some point between this age and the Rise of Ganon the Mirror washed up not far from Gerudo territory.

After the battle at Hylia Marine, Princess Nylin Zelda I handed over Veran's Fairies, the Sheikah traitors and Twinrova's armies (who marched against the Warlords orders), the Warlords then decided to banish these prisoners of war through the mirror.

The decendants of these criminals eventually evolved into the Twili.