Crucible can have different definitions.
The other two that google provided was:
"a place or occasion of severe test or trial."
"a place or situation in which different elements interact to produce something new." (note, this isn't the type of crucible you're talking about, this is more of a symbolic meaning of what you're talking about, as if you are comparing something TO a crucible, but it's a phrase that was used enough to change the meaning of the word
It could mean that that item is the product of Mikael combining two things, or to achieve that, you would need to pass a test that Mikael set up, and that is the fruit of the effort.
Crucible is one of those words that means so many flexible things that you can come up with a number of backstories that might make sense, it doesn't have to be a literal crucible.
Because while we're at it, almost none of the swords in the game look remotely realistic, and some of them are hilariously inaccurate , like 
Here's a picture of what a real scimitar looks like: http://windlass.com/wp-content/themes/windlass/product-imagery/swords-knives-daggers/scimitar/scimitar-2.jpg
It's a fantasy game, so I don't really care if it looks oddly proportioned, it's just the art style, but pretty much every single sword in the game is oddly proportioned or has a very impractical design that would get you killed in real life.
for example has a hilt that is much too thick, and way too short. If it were a real sword it looks like it would only be 2 or 3 feet in length in the first place.