Great Protector lived on the island, Caldera Cay. At some unknown time, Great Protector became the guardian of the human tribe that inhabited Caldera Cay, the Defenders of the Wing. Sometime in her younger years, a great earthquake shook the Cay, and the lava stopped flowing. She almost starved, but after much prayer by the Defenders, the lava started flowing again and her life was spared. For forty years, she protected the Defenders of the Wing by preventing the volcano on the island from erupting, which would destroy their village if it did. She did this by eating the lava. ("A Matter of Perspective")
One day, in the episode "Defenders of the Wing, Part 1", after being led by a supposed truce with the Dragon Hunters, the Dragon Riders arrived on Caldera Cay. When they got there, the riders saw a village, but were kidnapped by the Defenders of the Wing. The Defenders of the Wing believed the riders to be Dragon Hunter spies, but eventually Hiccup was able to convince their queen, Mala of the truth. However, the truce was all a ruse by Viggo Grimborn to lead the riders to Caldera Cay for use as a distraction, while they themselves went into the volcano and knocked out the Great Protector with Dragon Root arrows and kidnapped it. When Mala and Hiccup noticed that the volcano had excess lava spilling out, they walked to it and discovered the ordeal.
The Eruptodon appeared when Viggo and Ryker were visiting it in the cell bay on their Dragon Hunter ship. When Viggo's armada arrived at the Edge, they released the dragon to the mountains of the Edge. When the Eruptodon would start eating the volcano, at some point it will erupt and sink the island. However, when Hiccup and the riders, along with Mala came to the Edge the Hunters attacked. Later, they overpowered the Hunters and rescued the Edge by making a lava trail back to Caldera Cay, with the help of the island's wild Gronckles living on the Edge.
The Eruptodon briefly appeared when Snotlout was tested in Saving Shattermaster. Throk signaled the Protecter to fire lava blasts in order to make the rocks for the first trial.
In "Out of the Frying Pan", the Protecter laid an egg for its offspring to train it to become the next Great Protector. However, the Eruptodon was too weak to fly the egg to the lava chamber for the ritual of birth. So Mala, Hiccup, Fishlegs, and Meatlug flew into the lava chamber and with the help of the Fire Terrors the ritual ended to be a success. The Protector later gained enough strength to save Throk from the lava.
In "Shell Shocked, Part 1", When Ryker and the Shellfire attacked the island the Eruptodon gave the riders permission to take her offspring away from the Hunters, to keep it safe.
In "A Matter of Perspective", when the Great Protector became too old to protect Caldera Cay, Mala admitted that it was time for her to go to Vanaheim, a place where old or injured dragons live their last days. In secret, Ruffnut and Tuffnut followed her. When the riders came looking for them, the twins were watching the Eruptodon. Later, when the riders were trying to escape, she helped them, distracting the Sentinels, before entering a cave where she presumably died.
The symbol on the Defenders of the Wing boat sails is of the Great Protector.