Soul of Starlight

Souls of Starlight start off with a 0.01% chance to drop from any mob after Hardmode has been achieved and before the Moon Lord is defeated.

Upon defeating the Moon lord the drop rate increases dramatically from 0.01% to 24.50%, usually in quantities of 3-5

Crafting
Soul of Starlight is only required to craft 2 items, Skybreaker Omega, and Starlight Arrows.