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.

Soul of Flight is dropped by Wyverns, Soul of Sight by The Twins, Soul of Light by enemies in the underground area of The Hallow, Soul of Night is dropped by enemies in the underground area of The Corruption. Soul of Fright is dropped by Skeletron Prime, and might is dropped by The Destroyer. 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.