Nebula Fragment

Nebula Fragment is a Hardmode end-game material. It is dropped from the Nebula Pillar Boss during the Lunar Event. Nebula Pillar usually drops around 10-15 Nebula Fragments upon death in normal mode, and 20-50 in expert mode.

Trivia
Nebula Fragments are similar to the souls of the anime Soul Eater

Fragment Nébula