A typhoon that raged on in the Sullen Sea caused three Seadragonus Giganticus Maximus to stir from their deep slumber on the ocean floor: the Green Death, an unnamed Maximus, and the Purple Death. They eventually washed up on the Isle of Berk shortly after Hiccup and the other young Vikings were banished from their tribes following their tests on Thor'sday Thursday. The first two Sea-dragons washed up on the same shore together, the Long Beach, which eventually drove the Green Death to eat its smaller companion. The Purple Death washed up on a shore a ways off from the other two called the Unlandable Cove and is not discovered until Thuggory of the Meathead Tribe spotted it on the Highest Point while surveying the island as the Vikings formulated a plan.
Hiccup, the only Viking on the island capable of communicating with dragons in their native tongue, Dragonese, quickly came up with his own plan the "Fiendishly Clever Plan", to have the two remaining Deaths kill each other: by having the Hairy Hooligans' dragons harass and lure the Green Death in one direction while the Meatheads' harass and lure the Purple Death in the opposite direction. Hiccup planned to have the two collide with one another and kill each other. His plan worked as the two were led to Death's Head Headland and turned the corner of the cliff side separating them, crash into each other. The two Sea-dragons began viciously clawing and pummeling each other, resulting in the Purple Death's demise. Neither it nor its mountain-size carcass was seen again for the remainder of the book series.
The Purple Death is a very large Seadragonus Giganticus Maximus, even a little larger than the Green Death, but not by too much. He is, as his title implies, "a glorious deep purple". Due to his color, his smoke is also purple.
As with most dragons, the Purple Death is very vain with a superiority complex, and thus easily led astray by Hiccup's plan in which the huge dragon is taunted. He is so mad at being insulted, that he is willing to fight the Green Death to the death simply because he thought he was being insulted.