A few days ago, the community discovered these patterns drop from King Mosh in Un'goro, and soon after, Blizzard hotfixed them to require Core Leather, which can only be acquired from skinning Core Hounds in Molten Core.
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Blizzard has once again adjusted the reagents needed to craft the Devilcore items, this time removing the Core Leather requirement. With this change, players no longer need to wait for Molten Core to open before crafting these pieces.