Djinn's Curse

Desert Spirit is the only source, which means the Underground Desert in Hardmode. They are not thick on the ground and the drop is stingy, so plan on a deliberate farm rather than picking one up in passing.

Worn in the leg slot it gives you the Featherfall Potion effect with no timer and no brewing. Falls do no damage and you drift down slowly, so any vertical shaft becomes something you step into rather than something you rope down. For mining and building it removes an entire category of problem. It applies on mounts, inside minecarts and while gravity is reversed, with the Bee Mount as the one exception that ignores it.

Holding down cancels the effect and drops you at normal speed, which you want constantly, because floating is slow when you actually need to get somewhere. Holding up goes the other way and slows the descent much further, which is how you thread a narrow shaft or line up a landing. Learning to tap between the two is most of using this well.

The leg slot, entirely. It gives no defense at all, one of only three armor pieces in the game with that distinction, and equipping it forfeits whatever set bonus your armor was providing. In Hardmode that is a lot to trade for an effect a potion already covers. The lighter gravity has a second cost too: getting hit throws you much further than normal, so fights where positioning matters become harder to control, not easier.

Building, exploring and mining, where armor is irrelevant and never thinking about a drop is worth a slot. It also has one genuine niche in combat: it and the potion are the only things that prevent fall damage while Stoned. Since the two do the same job, running both at once buys you nothing, so pick one and free the other.