Tears of Light
In Twilight Princess, there are 16 Tears of Light or 12 in the HD version that must be collected within the Vessel of Light to return the Twilight-covered areas of Hyrule to normal. They are hidden inside Shadow Insects, found around the Twilight-covered area.
In each level of the Tower of Spirits, Link finds three Tears of Light. After collecting all three, Link's sword has the ability to stun Phantoms, giving Princess Zelda a chance to possess them. Once Link has the Lokomo Sword, he does not need the Tears of Light in order to do this.