Scardian is a Deadly Nadder who is covered in scars due to years of being mistreated by the Dragon Hunters. He is the alpha of a pack of scarred dragons, all of which were released by the Dragon Hunters as a plan to lure the Dragon Riders away from Dragon's Edge, however, this plan failed and Scardian and his pack remained free.
"Mistreatment by Dragon Hunters has left this Deadly Nadder with scars on both inside and out. It now leads a pack of very untrusting wild Dragons."
Before the episode 'Edge of Disaster', Scardian, along with a pack of wild dragons were captured by the Dragon Hunters. They were attacked and abused, left scarred both mentally and physically. After escaping, Scardian became the leader of the pack promptly after. They then settled on Forest Island. When Trader Johann sailed past the island, Scardian and his pack attacked his ship. They then came under attack by the dragon riders under Hiccup Horrendous Haddock III. During their run, Scardian's fellow dragons captured Fishlegs Ingerman.
When Fishlegs was captured by them, Scardian was wary and hostile towards the human, blasting him whenever he approached another dragon. It was not until he saved a baby Gronckle that Fishlegs gained the trust of Scardian.
During Hiccup's attempt to save Fishlegs, Scardian, together with a few more dragons from his pack, chased after him, thinking that Hiccup would attack them and was hurting Toothless. Hiccup showed him his flight suit, showing that his 'wings' made him no different from dragons. Scardian and his pack realized that the riders were of no threat and aided them in the battle at the Edge. They were then escorted back to the island by the riders.
Scardian is a teal Nadder with tints of pink, purple and yellow on its wings and a pale underbelly--a signature trait of Nadders. As a result of the mistreatment from the dragon hunters, he has been left with harrowing scars and marks on his body.
After being abused and mistreated, this Nadder has became incredibly aggressive and wary of humans, not hesitating to attack or warn anyone who comes to close to his pack. His hostility made him a powerful and protective leader of the dragon pack. This proved to be a great obstacle for Hiccup to tame him, but Scardian eventually gave in and proved to be a great ally when he saw Hiccup as one of his kind.