Some Challenge items are uniquely classified as Legendary Challenge items. Most of them have a secondary right-click attack, and are more powerful than other items at their tier. They are all dropped from their respective bosses with a 100% chance.
Prior to the 1.2.1.001 update, Legendary Challenge weapons' damage scaled as you progressed further into the game, and their abilities became upgraded upon defeating their respective boss. They also had a very small chance of being dropped by any enemy, with increased chances to drop from the bosses which are now their exclusive source. These features have since been removed from the weapons due to balancing issues, however The Community still retains similar scaling features.
Prior to the 1.3.2.001 update, the drop rate of Legendary Challenge items was 10% instead of 1%. However, they were significantly weaker as a result.
Prior to the 1.5.0.001 update, Legendary Challenge weapons were dropped by their respective bosses in Revengeance Mode or above, with a 1% chance.
Prior to the same update, Challenge items were known as Rare item variants and had a small chance of replacing their normal counterpart when dropped.
However, the Crown Jewel, Counter Scarf, and Teardrop Cleaver are exceptions to this, and were regular Revengeance Mode exclusive drops instead.
Similarly, the Burden Breaker only dropped from either the Dungeon Guardian during Hardmode in Revengeance mode, or from Skeletron Prime during the daytime in Revengeance Mode.