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The Nomadic Warrior's Cookbook 18 is one of the many Key Items found throughout Elden Ring, sold by an Abandoned Merchant in the Siofra River.
The Nomadic Warrior's Cookbook 18 is a Key Item Recipe Book that will unlock extra crafting recipes once you have obtained it.
This includes the following recipes:
Purchased From: The Abandoned Merchant located in the Siofra River, found by climbing up wooden scaffolding northeast of the Siofra River Bank and heading behind a waterfall to a secret chamber. It is sold for 6,000 Runes.