Smidvarg is a male Alpha Night Terror who lives on Dragon's Edge. When Hiccup Horrendous Haddock III and his team of dragon riders arrive on the island, they discover the Night Terrors. At first it is believed that they together were all one big dragon, but were then discovered to be a bunch of little ones. After Hiccup captured Smidvarg and an incident with Changewings occurred, Smidvarg and his flock became allies to the riders, who now act as "sentries" on the Edge.
Tuffnut discovers Smidvarg and his Night Terrors in "When Darkness Falls", when they flock together in the shape of a larger Night Terror, but no one believes Tuff when he tells them about "colossal" dragon he saw. However, the "large dragon" is later found to be split up into several tiny Night Terrors. Hiccup and the twins notice one white Night Terror coordinating the attack, and so, the twins capture it, thinking that it would solve the problem. Once captured, the name Night Terror was coined, though Tuffnut named the Alpha Smidvarg. The riders realized that the other Night Terrors were getting attacked by Changewings because, without their alpha, the other Night Terrors were easy prey. Hiccup then released Smidvarg so that he could go save his flock from the Changewings, which he did. Smidvarg and the flock later joined the Dragon Riders as sentries of the Dragon's Edge, and together they make up the shape of a Night Fury as thanks for defending then from the Changewings, to Toothless' excitement.
As shown in "Edge of Disaster, Parts 1 and 2" Smidvarg and his flock then assisted Astrid and Tuffnut in defending the Edge against Ryker Grimborn's Dragon Hunters when they attacked. However, Smidvarg was knocked out when Ryker shot him with one of his dragon root arrows.
He did recover and was later seen with Johann when he was left behind on Dragon's Edge.
Smidvarg made a brief appearance in "Family on the Edge", taking Snotlout's dinner.
He was also in Dire Straits when they made a Rumblehorn impression of Stoick.
As the leader of the Night Terrors on Dragon's Edge, it is his job to figure out ways to protect his flock, and even the entire island at times. He is very protective and gets agitated when the other Night Terrors are in danger. He is also willing to follow other's orders to protect the island, such as when he followed Hiccup and Toothless to lead the Fireworms away from Dragon's Edge.
Smidvarg is slightly aggressive, usually taking out his anger by roaring at Hiccup, since he's the one around at the time.
However, after earning his trust, Smidvarg is a loyal ally. He also has somewhat of a playful nature, as he and the other Night Terrors to take the shape of Toothless.
Intelligence: To show his gratitude to the teens by saving his kind from harm he made his clan to change into form of Toothless. Being the leader of the Night Terrors he figure way to defend his home from the Fireworms to change into large Fireworm hoping they follow it which work out great.
He is quite attached to his flock 'The Gang' (named by Tuffnut). Now that they all share an understanding, he gets along well with the Twins and the other Riders, even hanging out with them at the base. Especially after the Twins defended him from the Changewings.