When Galbatorix's dragon, Jarnunvösk, was killed by Urgals, Galbatorix traveled back to the Dragon Riders' stronghold in Ilirea, where he was brought before the Council of Elders. He demanded another dragon and the desperation of his request revealed that he was insane. He was denied his hope and to exact revenge, he formulated a plan to steal a dragon hatchling.
With the help of Morzan, Galbatorix stole Shruikan after he murdered his true rider. Galbatorix then named the dragon Shruikan, a perversion of the usual custom, as dragons normally opted to choose their own names when they were old enough to understand this concept. He twisted the young dragon's mind with powerful Dark magic, so that Shruikan would serve as Galbatorix's new dragon. Galbatorix used Magic to artificially speed up Shruikan's growth, making him gigantic in size.
When Shruikan was fully grown, he was forced by Galbatorix and his servant, Morzan, to take down the Riders and kill his own brethren. Shruikan was spared in The Banishing of the Names because he was forced into servitude, unlike The Forsworn's dragons, who chose to help. By the time of the Varden's rebellion against Galbatorix, Shruikan was believed to be the only remaining dragon in all of Alagaësia.
Bond with Galbatorix
The bond between Galbatorix and Shruikan was a mangled and corrupted version of the natural link between dragon and Rider. Therefore, it is extremely likely that Galbatorix had control of Shruikan's Eldunarí, which gave Galbatorix control over Shruikan. The nature of the bond caused Shruikan to become an insane killing machine.
Shruikan was a black dragon of titanic size. According to Inheritance (book), Shruikan had a pale blue-white iris, the color of a high mountain glacier. He was larger than Glaedr and told to have spikes as thick as tree trunks and made the ground shake for more than a mile around him with every step he took. He was eventually killed in Galbatorix's throne room because the room was too small for him to adequately move, allowing for Saphira and Thorn to hold down his head so Arya could stab him through the eye with the Dauthdaert, Niernen, killing him.
Shruikan went insane and was consumed by hatred due to having been controlled, corrupted and tortured by Galbatorix his entire life. His loyalty to Galbatorix is questionable, but Shruikan seemed to take tormented pleasure in fighting for him. When he made his first appearance in Inheritance, Elva noted that he was the most unhappy creature she had ever met. All she could sense within him was pain and anger, due to his madness. She goes on to say that if he could, he would destroy everything in his path and burn every plant until nothing was left. Elva had admitted that he was too far beyond sanity to be helped and Galbatorix said that he was angry with everyone. Eragon too could sense the malice in Shruikan's gaze. Galbatorix also mentioned that if Shruikan was woken up, he got irritated and tended to eat people.
Shruikan's first actual appearance was when the Varden were outside the gates of Urû'baen. Galbatorix used Shruikan to demoralize the Varden by revealing his immense size and power. He came from Urû'baen and then flew around, unleashing a jet of flames before flying back to Urû'baen. It was impossible to tell his size, but Eragon guessed that he could have been the biggest dragon he had ever seen, even exceeding Glaedr's height.
Shruikan next appeared when the four warriors, Eragon, Arya, Saphira, and Elva confronted Galbatorix. He was mistaken for a large curtain as he lay behind the King's throne. When he awoke he made a challenging glance across the group and a challenging growl at Saphira personally, as his growl was much louder than hers.
When they broke free of Galbatorix's magic, Thorn attacked Shruikan, although he only succeeded in getting injured. However, the two dragons were able to get Shruikan's neck down almost to the ground with their combined weight and due to the fact that the room was too small for him to lift his neck up. As the two dragons did so, Shruikan breathed his black fire and almost succeeded in incinerating Arya, who had the Dauthdaert in her hands. Because Shruikan couldn't move his head, Arya was able to stab Shruikan through the eye with the Dauthdaert, Niernen. With that, Shruikan's miserable life finally came to an end. Before this, Elva remarked that Shruikan's life was so miserable, she could only feel his pain. She said the best thing they could do for him was to put him out of his misery. Shruikan's corpse was stripped of all flesh by Galbatorix's suicide spell. It is probable that his skeleton was destroyed when the rest of the throne room collapsed on it.
Shruikan appeared briefly at the very end of the Eragon film (seen to the right). He was seen blowing fire in the last shot of the film, after Galbatorix cuts a map in half, which reveals the dragon hiding behind. There was no mention that Shruikan was not Galbatorix's original dragon, but a stolen one.
During the film, Shruikan's growling is slightly audible during a scene where Galbatorix pushes Durza against the map of Alagaësia. It is very faint but noticeable. Because the movie was released long before the publication of Inheritance, Shruikan's appearance in the movie is inconsistent with his description in the novel; notably, his eyes are golden, and judging from the apparent size of his head he appears to be much smaller than he is described as being in the books.
Fans speculate that while Galbatorix was capturing Eldunarí from dragons during the Fall of the Riders, he spent years trying to make them bend to his will. Therefore, it is wondered if Galbatorix had Shruikan's Eldunarí. This action would have been easier for Galbatorix if he had possession of Shruikan's Eldunari. However, it is questionable if Galbatorix had enough hold over him to force him to disgorge his heart of hearts.
With the publishment of Inheritance, it was revealed that Shruikan was beyond any hope of being saved. Eragon asks Elva after his appearance if she was able to feel anything from the massive dragon, to which she replies that she can only feel great pain and anger because of everything Galbatorix had done to him. Eragon then asks her if there's any way to reach him, but to his dismay, Elva tells him that there is none and that the best thing they could do was put him out of his misery.
It is unknown whether or not Shruikan's Eldunarí survived the events of Inheritance, or if it was destroyed in Galbatorix's blast-however, it would be likely that, seeing as Eragon and co. managed to gather all of the other Eldunarì in Galbatorix's possession, that therefore if Galbatorix was keeping Shruikan 'under a leash', then Shruikan's Eldunarì would be found with the rest.