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 harmful fog. 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.

Wind Waker
In Wind Waker the Master Sword is infused with light after you defeat the Earth and Wind Temples.