Souls of Night
This unique currency appeared when the Terraria crossover patch was released. They were removed in the official release of the game.
The Souls of Night previously allowed you to buy special weapons from the Wayfarer (Outdoor Hub/Town) that relate to Terraria in some way, and were only obtainable from Forest Biome and Blood Moon Incursion maps.
The Soul of Night is an item from Terraria.