Certain artifacts can be crafted using ingredients obtained from Peerless bosses. These items all require a "major" ingredient, only one of which can be obtained per battle.
They require very high levels of skill to create. Tailors and Blacksmiths are advised to use power scrolls (gathered via BODs) in order to improve their chances. Talismans can also tip the odds in your favour - you'll want as much help as you can get, considering that failure could potentially destroy ingredients worth over hundreds of thousands of gold coins...
Aside from Peerless ingredients, Craftable Artifacts sometimes require special resources specific to Mining, Lumberjacking and Fishing (such as gems), along with more mundane resources.
The character who wishes to create the item must have first read the relevant recipe scroll - these are, in some cases, considered more valuable then the items themselves (as they are far rarer then the components). The only exceptions are the Darkwood Armor pieces.
Note that Runic Tools cannot be used to further enhance craftable artifacts, and typically cannot be created Exceptionally.