Groundsplitter

Appearance and Personality
Alvin's dragon is a Whispering Death, and it looks much like all other Whispering Deaths that have been seen in the franchise. This Whispering Death has the typical personality of its kind; highly aggressive, violent, and dangerous. With these character traits, it seems to be a perfect match for Alvin. With dragons like this at his command, Alvin has become far more dangerous. It is obvious now why the "Riders" of Berk will have to become "Defenders".

Dragons: Riders Of Berk
In the episode We Are Family Part II, Alvin finally gets his own dragon, a Whispering Death. Alvin had captured this dragon caged it along with the other dragons. With the help of Mildew, he manages to trick Hiccup into revealing his dragon training secrets. He tames it along with Mildew and its name is yet to be revealed (if it has one).

Dragons: Defenders of Berk
Alvin's dragon does make a physical appearance in Defenders of Berk, however, it could possibly be the mother of the eggs the Outcasts placed in a tunnel under the Berk village.

The babies began attacking the village not long after hatching. The Whispering Deaths attacked at episode four of Defenders of Berk Tunnel Vision.

In Cast Out Part II, it is revealed that the Outcasts have at least two Whispering Deaths, one being the Screaming Death's mother. They were all released as part of Hiccup's plan to save Stoick. All of them leave after the Screaming Death finally finds its mother.

Trivia

 * This Whispering Death is the first dragon trained by an Outcast.
 * Like all dragons, the Whispering Death was captured and caged to be trained.
 * It first appeared in the episode We Are Family Part II.
 * This is the first dragon that Alvin has trained.
 * Hiccup was nearly able to train this type of dragon, but when it saw the Academy dragons, it turned away. Alvin was able to train this dragon easily, since there weren't any other dragons around, and since this dragon was not bent on avenging a grudge. Still training a Whispering Death is no small feat because Hiccup was nearly killed when he attempted to train a Whispering Death.