The largest, strongest, and deadliest species of dragon.
They can lie dormant for centuries and are "20 times as big as a t-rex".
The only thing they fear is each other. Their fights can end in complete bloodshed.
In how to train your dragon
There were three sea dragonus giganticus maximus in the book.
The first was averagely enourmous(by their standards of course).
The second, the green death(called red death in the movie) ate the smaller one who "was full of himself".
The biggest, the purple death, was tricked into helping the hairy hooligans and mogadon meatheads.
The fight that ensued was extreme. In the end, the purple death died and the green death was seriously injured.
When Hiccup Horrendous Haddock The Third got stuck in the dragon's throat, his dragon, Toothless, saved him and Hiccup bloked it's fire holes, a dragon's source of fire. It exploded.
In the movie, the green death was renamed the red death. It died when Hiccup and Toothless managed to set his wings on fire and he fell with an explosion.