Hallowed Bar

Hallowed bars are ingots used to craft high-end armor/weapons in the game. They are dropped by the Hardmode bosses, namely The Twins, The Destroyer, and Skeletron Prime. This can make them rather hard to obtain or farm.

Before 1.2, crafting Hallowed Armor and gear required Souls, as well as the Cobalt, Mythril, and Adamantite versions of the item in question. With the release of 1.2, crafting Armor and weapons such as Excalibur has become exponentially easier.

Dropped By
Skeletron Prime, The Destroyer, The Twins

Tips
If you are farming hardmode bosses for money, crafting Gungnirs and selling them is the best way to make money, as their base price is 30 gold.