Master Sword Infused with Light

In Twilight Princess, the Master Sword becomes engulfed in the light of the Sols. This enables Link to slash through darkness as well as defeat most shadow monsters in a single hit. The sword is also capable of clearing away dark fog that causes Link to revert back to wolf form. The upgrade is obtained in the Twilight Palace. This upgrade is only available while in the Twilight Realm.

Midna notes that the Sol's light being transfered to the Master Sword is the work of the gods of the Twilight Realm, and that they, too, are on Link's side.

The Wind Waker
In The Wind Waker, the Master Sword glows with the power to repel evil after Link defeats the bosses in the Earth and Wind Temples.