Soul of Light

Soul of Light is a Hardmode crafting material dropped by most enemies killed in the Underground Hallow (in the Underground and Cavern layer) with a 1/5 (20%) / chance. This also includes enemies not native to the biome such as Granite Elementals, but Meteor Heads do not drop Souls of Light.

Crafting all items, including a full stack of five Light Discs and excluding the boss-summoning items and, requires 109 /   89 Souls of Light. One of each boss-summoning item and a Key of Light require an additional 24 / 14  Souls of Light.

Tips

 * One of the best ways to farm for Souls of Light is to go to the Underground Hallow, and deep underground where lava spawns frequently, and place a few Water Candles. The enemies the Underground Hallow spawns do not go through blocks (except for Enchanted Swords) and are not special in any real way, so they get affected by the lava and drop Souls of Light. This is also effective for any other item dropped by Underground Hallow enemies.
 * Another easy way to farm these is to go to the Underground Hallow and summon King Slime using the Slime Crown. All of the Blue Slimes he summons have a chance to drop Souls of Light (this method will also work for the Soul of Night). (Patched in 1.4)

History
Seele des Lichts Âme de lumière