Tormentor is the Catastrophic Quaken that lives in Dark Deep. Initially thought to be very hostile, he was discovered by Fishlegs Ingerman to have only been lonely and scared. Fishlegs was able to calm Tormentor down and help him make friends with the local Gronkles .
Hiccup and Fishlegs discovered the fearsome dragon and were forced to retreat back to the Edge. Back on the Edge, Fishlegs began to train with Snotlout to fight Tormentor and chase him away from Dark Deep.
During the final confrontation, Meatlug defended Fishlegs from Tormentor, putting the two Boulder Class dragons in a staredown. Meatlug was able to see that Tormentor meant no real harm, but was actually acting out of self-defense. Once Meatlug showed him kindness, Tormentor was accepted by the Dark Deep Gronckles and began living in peace with them. After this, he appears to have a small crush on Meatlug, though it is unknown if she returns his affections.
One day, when Fishlegs needed more Gronckles for making Gronckle Iron, the twins and Snotlout journeyed to Dark Deep. There, they found the Catastrophic Quakens caring for the Gronkles. When the three riders got closer to the dragons, the Quakens chased them away. The twins suggested that Snotlout would smash his axe on the ground to create vibrations and calm the Quakens down. He did so, but the dragons responded aggressively and began chasing him. Taking the opportunity given by the situation, the twins lured the Gronkles away, leaving Snotlout by himself with the Quakens.
Later, at Dragon's Edge, the Dark Deep Gronckles got stuck under a few boulders, with a flow of lava closing fast. The Catastrophic Quakens arrived and, seeing the situation, the began smashing the other side of the volcano, releasing the presure and stopping the lava flow. They then freed the Gronckles and both species returned to Dark Deep.
Tormentor was a bit of a bruiser, bullying and chasing away the Gronckles of Dark Deep. However, Fishlegs discovered he was just being defensive because he was all alone.
Thanks to the help of Meatlug, Tormentor has become very friendly to the Gronckles, as he was willing to share his home with them to make Dark Deep a more peaceful island. He even showed forgiveness by breaking large boulders to feed his new friends.
Tormentor looks like any other Catastrophic Quaken. He has a grayish-silverish color on his scales, he has a Gronckle-like shape, and has a mouth with a bottom part that can separate in two. Tormentor is also shown to have orange-yellowish eyes as well. He contains many spikes on his head, with many on the top and on his chin, as well as a few small spikes on both of his wings.
In the School of Dragons expansion pack Battle for the Edge, Fishlegs and the player encounter a Catastrophic Quaken on Dark Deep. It may be Tormentor, although it has a different coloration.