optimum tantrum FUN in 7 easy steps!
1) turn off all industries. Let all dwarves idle.
2) make one enormous grand dining room, and set it as a meeting hall.
3) wait 2 game years this way.
4) NOW, turn industries back on
5) Train an extensive military. Let no dwarf not know how to swing an axe or hammer!
6) send a recon expedition into cavern layer 3.
7) when they find something horrible, tell them to kill it. (They will likely fail.)
As everyone's favorite darling, Urist, gets felled by Osom Gulakularth the foeted troglodite, or worse, some horrible forgotten beast, the others in the fortress will freak out.
It is important that multiple people die in the recon mission. Send a full squad. Make sure they all die.
Once the freakout begins, they will begin to tantrum. Those that don't immediately will shortly, as they get subjected to killed pets, art defacement, getting punched, etc. Because everyone is friends with everyone else, this extended relationship causes an implosion of morale. In just a very short time, they will all be angry and murdering each other, and angry that their friends are being murdered.
Let it play out. Rotting corpses increase the intensity of the pheomenon. Ghosts further intensify the problem, especially since there is a good chance some will be killer ghosts.
At the end of it all, your fortress population will be decimated, ghosts and dead bodies will be everywhere, and the few remaining survivors will be either so traumatized that they literally jibber and do nothing else, or will have lost all sensitivity, and become emotionless husks.
Cleaning up afterward can be challenging, and survival of the spiral is not garanteed.
The fortress' networth will be mostly intact, so very large seiges will continue, even if you only have 4 dwarves left.
Tantrums are great fun!