Soul of Night

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

Crafting all items, excluding the boss-summoning items and, requires 97 /   89 Souls of Night. One of each boss-summoning item and a Key of Night require an additional 24 / 14  Souls of Night.

Tips

 * An easy way to farm these is to go to the Underground Corruption/Crimson and summon King Slime using the Slime Crown. All of the Blue Slimes he summons have a chance to drop Souls of Night (this method will also work for the Soul of Light). (Doesn't seem to be working in 1.4)

History
Seele der Nacht Âme de nuit Эссенция ночи 暗影之魂