When a dwarf has been unhappy enough, he'll decide that enough is enough and he'll throw a tantrum. Tantruming dwarves will cancel any job they may be doing at the moment and will start misbehaving, which includes throwing items around, starting fist fights (with the possibility of severely injuring other dwarves), toppling and destroying buildings or hurting pets. This is of course illegal, and they will be imprisoned or flogged for their behaviour. But even though they are punished, if their punishment is more harsh than usual, their death (or the results of their misbehaviour) can cause another wave of tantrums, and your dwarves will begin a process known as a tantrum spiral which will almost certainly destroy your fortress.
Tantruming may be a warning that the certain dwarf may go insane unless something makes the dwarf happy.
To make tantrums less likely, see the article on keeping your dwarves happy.
(This requires lots of friends, married couples, and the oh so common bad luck.)
- Dwarf A, a craftsdwarf, gets the urge to build a mysterious construction, but unfortunately there are no 'body parts' or shells to be had.
- A goes berserk, kills dwarf B, and wounds dwarf C. A is then struck down by dwarf H, making a unhappy thought for A's friend dwarf E.
- B's loved one dwarf D, and friends E and F, get a strong unhappy thought.
- D, going to help C, throws a tantrum. He kills C in his rage, giving another unhappy thought to C's friend F.
- F, now horribly unhappy from the death of two of his friends, throws a tantrum destroying E and D's beds.
- E, now very unhappy from two of her friends dying as well as losing her bed, goes melancholy and commits suicide by jumping down the well, giving an unhappy thought for D and J; and contaminating the only water source.
- D, despite the happy thought for fighting (and killing) C, is overwhelmed by unhappy thoughts and tantrums again. This time he destroys a bridge, drowning F in the moat (as it has no ramps), and then punches dwarf G.
- F's friends, I and J, both get an unhappy thought.
- E's body rots, causing J to tantrum. He punches G, hospitalizing him.
- D once again punches someone, this time I - cutting his lip.
- Without fresh water, G dies a slow death of dehydration.
- I, even more angry due to being punched, punches D back.
- D, finally being overwhelmed, goes stark raving mad.
- I, angry about being D's punching bag, punches J, and J punches H out of anger.
- I, Unable to wash his lip with clean water combined with all of the rotting corpses, succumbs to infection.
- D dives into magma, creating a unhappy thought for D's friend, H.
- H, in the wake of death and Miasma finally goes berserk and finishes the fortress off.
Fortress at the start: 10 "normal" dwarves.
Fortress after tantrum spiral: 9 dead dwarves, 1 berserk dwarf (likely soon to die from starvation/dehydration). Causing fortress-stopping Fun.
As a result of not having the right 'body parts' or any shells, your fortress falls. The main thing that started this mess was Dwarf B's death. And bad luck, isn't losing Fun?
 Damage Control
It is possible to survive a tantrum spiral, but it's going to take some serious damage control.
- Prioritize making alcohol, coffins, and food over everything else. Stop any unencessary work.
- Bury all that die, burial lessens the unhappiness of death, and prevents miasma.
- Handle the most unhappy dwarves with special care. Giving them additional rooms and furniture could slow down their happiness decrease. Relieve them from work and let them meet their friends, pets and relatives if they have any. Expedition leader/Mayor/Baron etc. could sometimes give them consolations, or be yelled at, so make him available for meetings. If situation around a certain dwarf seems unresolvable, try to isolate him while you can, preferably with walls.
- Move military inside to kill anyone that berserks. You'll die if an ambush hits you during the spiral anyway.
- Create Jails by placing rope or chains in small rooms. Under Build (b) Restraint (v), then select the restraint using (q) and select Use for Justice (j), much like you would when placing a bed and making a bedroom. This will place people miss behaving or throwing tantrums in jail. Then if they do go berserk, they will be restrained away from everyone else till they calm down. When a fortress starts Tantrum spiraling make as many Jails as needed.
- Also, if, by a stroke of luck, some already-happy migrants come, then they will add to population and you will now have dwarfs who will do work without being interrupted by the need to destroy everything.
- Door abuse inside your fort can easily stop interaction between dwarves and therefore can lessen the impact of a tantrum spiral. You will need a lot of doors though, a lot of micromanagement, and quite a bit of planning before the tantrum spiral.
 Tantrum spiral immunity
Yes, it is possible to make your dwarves resistant or even immune to tantrum spirals. This is done with the "combat hardness" variable.
The point is to expose your dwarves to enough violence and bloodshed so they acquire the description "doesn't care about anything anymore". Dwarves with this trait tend to barely react to anything, and they will most likely go through a tantrum spiral without tantruming.
This trait tends to be seen more on military and hunters, but it can visibly be earned by other methods that getting each of your dwarves to kill enough creatures to earn it. See this thread for more information :http://www.bay12forums.com/smf/index.php?topic=85641.0