Niffelheim
33 Achievements
GOG
Blacksmith
Craft 100 items in the Forge
28.7%
How to unlock the Blacksmith achievement in Niffelheim - Definitive Guide
This should come with time if you are going for the full completion of the game. However, if wanting to get this done sooner rather than later, the quickest way is to mine coal and copper/iron in the first 15 floors of the mines inside your base. Then smelt the coal and minerals into ingots using the forge you construct on the far left side of your workshop.
The Forge is one of the 4 work stations you'll build in your Castle/keep (it's the furthest left). You'll most likely get this trophy naturally as you'll be crafting the ingots here. These are required not only for weapons and armor but also upgrades for your work stations and the castle itself. You'll come pretty close to 100 with just the upgrades alone so if you don't have this by the time you've reached 100 days, just keep crafting Coal until you reach 100. This item only takes Timber to create so you'll be able to get a lot in a short time compared to the other craftable items. When you craft your 100th item, the trophy will unlock.