Shadow Chest, Terraria Wiki

Shadow Chests are chests found rarely in The Underworld. These can contain semi rare and powerful items such as the Sunfury, Flamelash, Dark Lance, and the Flower of Fire. To open a Shadow Chest, the player will need a Shadow Key, which can open an unlimited amount of Shadow Chests without being consumed. Once a Shadow Chest is unlocked and emptied, it can be picked up after being broken down with a Pickaxe. Shadow Keys can be obtained from chests in the Dungeon, or rarely in underground chests.

Wiki says that these chests will open once skeletron is killed

Boomstick - Terraria Wiki

This is the most stacked shadow chest I've opened to date. Not

Molten Armor, Terraria Wiki

Category:Loot items - Terraria Wiki

Don't dig up - Official Terraria Wiki

Sunfury - Terraria Wiki

1.2 Changes - Terraria Guide - IGN

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Shadow Chest 2 by Terraria247 on DeviantArt

Dungeon - Terraria Wiki

Chests - Official Calamity Mod Wiki