Celestial's Particle is a material added by AlchemistNPC mod.
It's used in crafting any 2 of Celestial Fragments and many other stuff in the mod itself.
To obtain Celestial's Particle, you need to defeat Dungeon Guardian.
On Normal mode, DG will drop 5 - 10 Particles.
With Expert mode, it will drop 15 - 20 Particles.