Strange Mood
Periodically, individual dwarves are struck with an idea for a legendary artifact and enter a strange mood. Dwarves which enter a strange mood will stop whatever they are doing and pursue the construction of this artifact. If they do not manage to begin construction of the artifact within a handful of months, they will go insane and die soon afterward.
Overview
For the duration of the mood, the dwarf will claim a workshop or furnace related to the skill that the mood affects, kick out any dwarf who was using it, and render it otherwise unusable until the mood has ended. If a moody dwarf does not claim a workshop, it is because the appropriate workshop does not exist.
After claiming a workshop, the dwarf will set about collecting the required materials for their artifact. The required items can be seen by examining the claimed building. If the dwarf remains idle inside the workshop, it's because they cannot find the right material.
Once all materials have been gathered, the moody dwarf will begin construction. Upon completion the dwarf will create a semi-random artifact related to the skill affected and gain legendary (or higher) status in that skill (unless the mood type is possessed).
Types of Moods
Fey - the most basic strange mood.
Secretive - the same as fey moods, except a secretive dwarf will sketch pictures of their required materials instead of clearly stating their demands.
Possessed - dwarves have cryptic material requests, and have the unfortunate distinction of not receiving any experience upon successful construction of an artifact.
Fell - dwarf will always take over a butcher's shop or a tanner's shop. They will then murder the nearest dwarf, drag the corpse into the shop and make some sort of object out of dwarf leather or bone. Only unhappy dwarves may enter a fell mood.
Macabre - similar to fell moods, but the dwarf will not murder a fellow dwarf.
Success and Failure
Successfully creating an artifact grants a very strong happy thought (enough to make the creator totally ecstatic for several months) as well as granting the creator partial immunity to insanity.
If you can't provide the desired workshop and all the required component materials within a couple of months, the dwarf will go insane and die soon afterward.
A dwarf who is stark raving mad, melancholy, or catatonic is harmless to others (until they die and start a tantrum spiral), but a berserk dwarf will attack other dwarves and possibly pull levers at random. You may want to station a squad nearby or assign a few war dogs to the dwarf on the chance that they will lash out.