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Gather all the ingots in the Inferno Vault and take them to Hell's Forge.

Chapter 3: Inferno Vault has four ingots to collect which can then be used in the Forge to create a Creature Core which will unlock the achievement. The Forge itself is behind the Trickster Door in this level (see "Gatekeeper" for that location). The locations of the ingots are as follows:

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1. The first one is literally directly in your path going down the first set of stairs in the level.

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2. After fighting a mage enemy, you'll notice a section of the level below you to the left that you can drop down to. From here, glide over to a new platform further to the left to find the second ingot.

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3. After fighting two mage enemies, you'll unlock one of the glowing blue orbs to fly up to a higher area where a Trickster Key can be found. Instead of taking this right away (or if you did, backtrack to where the mages were), find a ledge you can jump to off to the left where the third ingot will be.

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4. After unlocking the Inferno Vault (using the bomb to push in one lever and doing the other yourself when playing solo - or coordinate with your co-op partner to do both at the same time), jump over a small gap to a new section on the right side of this area to find the final ingot.

As mentioned above, with all four ingots in hand, backtrack out of the Inferno Vault and into the Trickster Door just to the left to find the Forge. Interact with it to get your Creature Core and unlock the achievement.