Final answer:
The secret Serbian nationalist society that planned the assassination of the Archduke was called The Black Hand.
Step-by-step explanation:
The secret Serbian nationalist society that planned the assassination of the Archduke was called The Black Hand. The Black Hand was formed in 1911 and was also known as the Unity or Death group. They were responsible for the assassination of Archduke Franz Ferdinand of Austria-Hungary, which sparked the start of World War I.
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