This Whispering Death first appeared in What Flies Beneath, where it was revealed he and Toothless had a history, and not a nice one. He is a ferocious and merciless fighter, who was determined to kill Toothless. He is also shown to be quite intelligent, being able to figure out Toothless couldn't fly, and thus was an easy opponent. After Toothless defeated him, it is assumed both have moved on and consider the matter closed.
This Whispering Death appeared in episode fourteen (What Flies Beneath). He held a grudge against Toothless who had bitten him before Toothless and Hiccup became friends, and got into two fights with Toothless after he arrived on Berk. In the first one, he showed himself to the rest of Berk's dragons, but they all backed off when Toothless attempted to confront the Whispering Death by himself. Hampered by his inability to fly, Toothless wrestled with the Whispering Death until he was shot in the leg by one of the Whispering Death's spikes. In the second fight, Hiccup was prepared to help Toothless, but Toothless refused and tried to manage on his own. Without Hiccup, Toothless was unable to fly properly, so the Whispering Death knocked him out of the sky and used his fire to send him onto a ledge where the Whispering Death attempted to send Toothless into a canyon, which would kill him. Only by accepting Hiccup's help was Toothless able to defeat the Whispering Death, Hiccup deliberately forcing Toothless to accept his aid by jumping off the canyon himself, leaving Toothless to save Hiccup and accept his aid or let his friend die to save his vendetta. After he and Hiccup were able to down the Whispering Death using its weakness to direct sunlight, Hiccup convinced Toothless to spare his foe, which then fled back to its tunnel.
Toothless’ Nemesis appeared in Hiccup’s flashback in Darkest Night, in place of the Red Death if Hiccup had never shot down Toothless in the first place, as the dragon blows off Hiccup’s foot and Toothless’ tail fin.
It has never been revealed how, where, or why Toothless and this Whispering Death developed a grudge for one another in the first place. However, the game, Rise of Berk, hinted that the story behind their grudge might be revealed in a later season of Race to the Edge; this was later proved to be false.
He was formerly going to be called "Groundshaker" in Rise of Berk.
Brook Chalmers has revealed that Toothless Nemesis, will likely never return to see the truth of the dark history of the grudge of Toothless and his hatred of Whispering Death.
It's presumed that he is the same Whispering Death that was being fed by Mildew on Outcast Island and might be Groundsplitter's mate, but this is unconfirmed though.