A Desert Spirit's Curse, the projectile fired by Desert Spirits.
Desert Spirits have an attack pattern that is unique compared to most enemies. Upon appearing near a player, they will cast a single, colored projectile that attempts to home in on the nearest player, phasing through blocks and slowly gaining speed. After a certain amount of time, it will duplicate and explode. Similar to other casters, it will then teleport to a different location and repeat the process. It
should be noted that the projectiles themselves do not deal any damage, but their explosions do.
The Desert Spirit drops the Desert Spirit Lamp with a 2.5*1/40 (2.5%) chance, or the Djinn's Curse with a 3.33*1/30 (3.33%) chance, but never both. The Desert Spirit Lamp is rolled first, and if not dropped, the Djinn's Curse is rolled.[1]
Trivia[]
The Bestiary entry for the Desert Spirit: "Ancient magicks corrupted the minds of the once friendly desert djinns, releasing them full of hostility from their lamps."
↑Information taken from the PC1.4.3.2 source code, method RegisterMiscDrops() in Terraria.GameContent.ItemDropRules.ItemDropDatabase.cs. There may be inaccuracies, as the current PC version is 1.4.4.9.