The Purple Death is the secondary antagonist from the first book of the How to Train Your Dragon book series.
When the boys of the Hooligan and Meathead tribes saw the Seadragonus Giganticus Maximus, the Green Death, they will have a meeting on the high hill. One of the boys said that there is another dragon, the Purple Death. One of the other boys said that that is good news, since both of the dragons are big enough to fight each other. The boys got feathers and clothes. The feathers will be wrapped inside the clothes. The boys' hunting dragons will drop the clothes in front of the sea dragons. After that both dragons fought each other and the Purple Death got killed.
The appearance of the Purple Death is never shown in the books. (The picture above is just a random Seadragonus Giganticus Maximus.) It was mentioned that it is a purple Seadragonus Giganticus Maximus. He is also larger than the Green Death.
Not much of his personality is shown, but he is shown to be vengeful at another dragon, since he fought the Green Death.