The Whispering Death is a dragon that appears in the film continuity and the exclusive short, Book of Dragons. This dragon is seen when Hiccup reads the Dragon Manual, and they are classified in the Boulder Class.
The Whispering Death is similar in appearance to a Deadly Nadder, but with bulging eyes, a spiky face and spirals of teeth that line the entirety of its inner mouth. It has a snake-like body and a neck similar to a Zippleback's, but a Whispering Death has no legs, only lower spikes (which might have been used to crawl through their tunnels). Many consider them to be extremely vicious and often have nightmares after confronting this horror. They have gaping mouths, full of deadly, rotating teeth. They can also rotate their wings, presumably to assist in the drilling process. They look very similar to a Gulper Eel.
Whispering Deaths are extremely deadly, like most other dragons, and able to chew their victims after they are swallowed, using its many rows of rotating teeth. They tend to dwell in underground tunnels that they burrow themselves, resurfacing to attack any oncoming enemy/prey.
The Dragon Manual once stated that they were extremely dangerous and must be killed on sight. However, despite their fearsome reputation, they love to have their teeth brushed. In the 14th episode of Dragons: Riders of Berk- What Flies Beneath, Fishlegs observed that a Whispering Death's eyes are weak to direct sunlight, and so spends most of its time underground. Astrid states that it weighs about 2,500 pounds.
When a Whispering Death finds intruders in its tunnels, it will forcibly eject them out the nearest hole. This was done to Bucket, but later on when it did the same to Hiccup, it also tried to kill him, even pursuing him above ground; this may have been because while Bucket had merely fallen in, Hiccup had been caught running along its tunnel system. Perhaps the dragon considered this a deep intrusion of its 'territory'.
The Whispering Death has the ability to use its jaw to quickly travel through solid rock, giving it, arguably, the greatest jaw strength in the dragon world. Apart from that, the Whispering Death can breathe powerful rings of fire. It is incredibly strong and is also able to shoot its spines like the Deadly Nadder, which grow on its head and along the entire length of its body. With its long, thin body, its tail often makes a whipcrack noise as it flicks it about.
The six inner rings of teeth rotate in opposite directions of each other. When these teeth move, they make an eerie noise that sounds like many people whispering. Hearing "The Whisper", along with the ground rumbling, are often the only two warning signs that a Whispering Death is approaching from underground, unless it tunnels near the surface, tossing up earth and rock. When dragons are standing on the ground, they are apparently more sensitive to these signs than humans, becoming agitated before humans can even hear it. Toothless was able to detect the presence of one from Hiccup's room, which is on the second floor, even though it was not that close by.
It is apparently immune to the usual effects of dragon nip. When Hiccup attempted to use dragon nip to tame an attacking Whispering Death, sniffing it only made the dragon sneeze. It has also been shown to be unaffected by the sonic roars of the Thunderdrum.
The Whispering Death hates sunlight, which is why they spend their time underground. Though its teeth can rip apart rock, it can't tear through steel plating, specifically cast iron. Apart from this, the Whispering Death has no known weaknesses.
In the Film
The Whispering Death is mentioned when Hiccup reads through the Dragon Manual for information on Night Furies. Apparently, this dragon is one of the most feared dragons, since Hiccup says its name with great fear.
The Whispering Death has a longer appearance in the short movie- Book of Dragons. It is mentioned and told about it's nature and characteristics. It is also shown when it attacks Bork and his girlfriend where it burns Bork to ashes.
The Whispering Death made its first appearance in episode 14 What Flies Beneath. This particular dragon held a grudge against Toothless due to some past confrontation that resulted in Toothless leaving a distinctive scar on the Whispering Death's hide. Although Toothless attempted to fight it on his own, his inability to fly put him at a disadvantage. When the Whispering Death nearly killed Toothless by knocking him into a canyon it was only after Toothless accepted Hiccup's help that the Whispering Death was defeated. After it was driven away, the holes it made through it's burrowing were blocked up with boulders.
In episode 20- We Are Family Part II, another Whispering Death makes a appearance (it's not confirmed whether it is the same dragon which appeared in episode 14). The dragon has been seen caged up in Outcast Island throughout the episode. Alvin had attempted to get Hiccup to train it along with two other dragons, but Hiccup had escaped on a Deadly Nadder leaving Alvin frustrated. In the end of the episode, Alvin finally learns to train a dragon, as he is seen training a Whispering Death. Thus, Alvin achieved his goal. It is very likely that this Whispering Death will make more appearances, since he is now Alvin's dragon.