Souls

Souls are crafting materials that dropped by certain enemies in hardmode. Souls not affected by gravity and thus stay floating where they are dropped. Oddly enough, it has been observed that certain enemies will drop both Lights and Nights, based upon their proximity to Pearlstone or Ebonstone, respectively - Armored Skeletons and Skeleton Archers are a good examples of this for any who would want to test out this observation for themselves. Also, it seems that no souls of light or night are dropped on the surface. Cavern layer or deeper is recommended. In rare cases, where underground hallow and corruption meet, hallow enemies might drop Souls of Night and vice versa, depending on which biome the game decides you are in.