Which mythological artifact was marketed to ancient consumers as having the unique selling proposition (USP) of being able to render its wearer completely invisible?

Throughout history, mythological figures have demonstrated remarkable product positioning and storytelling that has kept their 'brands' alive for millennia. Even in the ancient world, successful myths spread through strategic 'marketing' techniques and compelling brand narratives that modern retailers would envy. Test your knowledge about how ancient mythological products and symbols became enduring cultural icons through what we'd now recognize as brilliant retail and brand strategies.

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The Cap of Hades (also known as the Helm of Darkness)
Thor's hammer Mjölnir
The Golden Fleece
Excalibur