Where are the key wardens and where are my keys?
What are the locations of the monsters that can drop the keys and the plans for the infernal machine introduced in patch 1.0.5?
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According to Blizzard's article on the Infernal Machine:
Note that these only drop in Inferno, and you need a full five stacks of Valor. So while the KeyWardens will show up in easier difficulties, they won't drop them.
Additionally, each level of Monster Power present will increase the chance of the key dropping by 10%, so at MP10, each Keywarden is guaranteed a drop.
The plans are taught to your blacksmith, and only are needed once. After that, you just need the keys from the previous three acts, as using the Infernal Machine consumes it.