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.
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. But as of the 19th, Core Leather has been replaced with Essence of Fire, an item which can be farmed up from the Fire Elementals in Fire Plume Ridge.