Dark Shard

Two recipes call for it, and both also want its Hallowed opposite. The Onyx Blaster is the one worth the trip, an early Hardmode shotgun that holds up well past the mechanical bosses. The Dao of Pow is a flail with a fun gimmick that most players skip.

The drop is tied to your world evil reaching a desert, so this is not something you find by accident. Track down where the Corruption or Crimson has spread into sand, dig into the underground desert beneath it, and the mummies down there are your main source. Ghouls and the burrowing enemies contribute at a worse rate.

The efficient approach is to hollow out one large room and let the whole biome spawn into it rather than chasing a single enemy type around. Since you need both shard colors anyway, a Clentaminator cuts the job in half: convert one underground desert, farm it out, then convert the same room to the other side and farm it again.

The dark and light shards are the two halves of a taijitu, and the Dao of Pow puts the symbol back together.