The Red Death lives in a dormant volcano, having all other dragons bring food to feed it. The dragons drop off the food in the center of the volcano, leaving the food supplies with the Red Death. After eating, the Red Death emerges and dragons that come in its way become eaten, even if they have provided food. The Red Death has been shown to take malevolent parasitism to the extreme, for it has been shown to eat dragons who fail to bring food for it (most likely to make up for the food the eaten dragons failed to provide), which explains why the dragons greatly fear it, and none have tried to fight it (fearing that rebelling against the Red Death would result in becoming eaten), until Hiccup and his trained dragon, Toothless, conceive a plan.
Stoick the Vast got this information out of his son, and soon led a party to where Hiccup had explained, using a restrained Toothless as their guide.
Hiccup helped his friends tame their own dragons to rescue Toothless from the Hairy Hooligans who captured the dragon, in hopes of being lead to the Red Death. Hiccup allies his friends with these dragons: Astrid and her Deadly Naddder, Snotlout and his Monstrous Nightmare, Fishlegs and his Gronckle, and Ruffnut and Tuffnut and their Hideous Zippleback. Hiccup then shares a ride with Astrid, and they part for the Red Death's lair, where they'll find Toothless and the Hairy Hooligans.
The Hooligans had opened fire on the side of the volcano, freeing the dragons inside and waking the Red Death. The dragons had gone, flying away, in fear of the wrath of the Red Death. The Hooligans, though, were not aware of this until it attacked them.
Hiccup and Stoick later help free Toothless from his bonds, and together, Hiccup and Toothless bombard the Red Death with their friends. The duo lead the Red Death to the sky, into the clouds. The Red Death had lost track of them in the clouds, as they were using them as cover. Toothless shot the Red Death in a sneak attack, and then disappeared into the clouds; he repeated this multiple times, until the Red Death was enraged, and set the surrounding area in fire. Toothless' synthetic tail caught fire, and they had to hurry their plan.
Hiccup drove Toothless downward, and the Red Death followed close behind. Toothless dove as fast as he could manage, though the Red Death always strayed closer. Just before the Red Death was about to fire on them, Hiccup turned his dragon and Toothless shot a bolt of thunder into the Red Death's mouth, overheating it. With the straying embers off Toothless' tail, the wings of the Red Death caught fire, and burned holes into the skin. The Red Death lost control and crashed into the shoreline, exploding to its death on impact.
With the Red Death now gone, the dragons are now free and no longer have to raid the island of Berk for their food anymore, and the Hooligans were taught to live with them in peace.