Soul of Light

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

Crafting all items on the, including a full stack of five Light Discs, excluding the Boss-summoning items and Key of Light, requires 98 Souls of Light (89 on console/mobile). One of each Boss-summoning item and a Key of Light requires an additional 24 Souls of Light (14 for console/mobile), for a total of 122 (103 for console/mobile).

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 great if you need 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).

History
Seele des Lichts Âme de lumière