Soul of Night

The Soul of Night is a crafting material dropped by all enemies killed in the Underground Corruption. This includes stray enemies not native to the biome, such as Goblins and Meteor Heads. The chance of dropping a Soul of Night seems to increase with depth. Enemies in the Underworld are very likely to drop Souls of Night when under Deep Corruption. They can be easily farmed with hard mode gear by placing ~50 meteorite in the underground corruption and killing the heads that spawn as seen in this video