It's in the Dungeon, from chests., I greatly recommend Shadow/Crimson armor for melee users, Jungle Armor for magic users, and Necro Armor for ranged., They contain the chest AND pots, giving extra loot., The Underworld is a very dangerous place, and if you stay for much longer, you'll probably get killed...
Find the aforementioned Shadow Key.
Venture into the Underworld.
Look for Ruined Houses.
Once you have enough, swiftly retreat.
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