Light Shard

The Light Shard is a Hardmode crafting material used solely to craft the Dao of Pow. It has a 1/10 (10%) chance to drop from Light Mummies, Spectral Mummies, which spawn in Hallowed deserts. Also it has a 6.67% (1/15) chance to drop from Dreamer Ghouls in the  Hallowed Underground Desert.

Trivia

 * The Light Shard represents the "Yang" of the "Yin Yang" (Taijitu) symbol.