Every so often, a dwarf in this game will fall into a strange mood. They will seek out a specific workshop and begin gathering items from all over your fortress. When they gather all of the items they require, they will begin a mysterious construction. When finished, they will produce an item of unsurpassed quality, and your dwarves will hail its creation. If it’s the right kind of mood, your dwarf will advance to legendary status in the skill required to make that item.
Often, though, your obsessed dwarf can’t find a specific item that they crave, and they’ll eventually go mad. This is a huge incentive to develop your infrastructure; there’s no telling when a dwarf will suddenly take over a forge and demand uncut tourmalines and bat leather.