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The Missionary's Cookbook 2 is one of the many Key Items found throughout Elden Ring, sold by Patches first found in Murkwater Cave, in West Limgrave.
The Missionary's Cookbook 2 is a Key Item Recipe Book that will unlock extra crafting recipes once you have obtained it.
This includes the following recipes:
Purchased From: Patches the Untethered, a boss of the Murkwater Cave in West Limgrave, who you can spare and will open up a shop instead. It is sold for 800 Runes.