Soul of Night

The Soul of Night is one of the six souls that monsters can drop after the Wall of Flesh has been killed in the world. Any enemy (excluding those summoned by statues) killed in the vicinity of 200 Ebonstone Blocks or Crimstone Blocks  while in the stone layer may drop Souls of Night. Even a Corrupt enemy (such as the Corruptor) will not drop Souls of Night unless 200 or more Ebonstone Blocks are present. Enemies that do not spawn in the area, but simply followed the player there, may still drop Souls.

You will need a total of 102 Souls of Night to craft all items.

Potential Farming Methods
The player can farm Souls of Night by placing 200 Ebonstone Blocks in the stone layer (characterized by a background comprised of stone rather than dirt). Monsters killed in this area may drop Souls of Night.

Creating an artificial meteor biome (by placing 50 meteorite ore within sight) in an area where souls can drop can make farming them much easier, as Meteor Heads spawn very quickly and have only 26 HP. Using a water candle or battle potion will increase the spawn rate even more.