When confronting Hiccup, Drago Bludvist reveals that he was a simple man, but quickly change when his family was attacked and presumably killed by dragons, even loosing his arm in the process and replaces it with a mechanical arm. He briefly lived in fear after witnessing his family being killed by dragons. Later on, Drago Bludvist eventually became dark, violent and without conscience or mercy and has become a tyrannical leader, and learns how to control dragons and make them bow down to him. He controls them harshly, proclaiming himself as 'The Dragon God'. With this new power he wishes to conquer the world crushing anyone who is foolish enough to stand in his way. During his early conquests he finds a Bewilderbeast hatchling and decides to take it with him as a slave and abused it until it grew to full size and used it to gather a huge dragon army. Drago attempts to visit Berk where a gathering of chiefs occured in the Great Hall including Stoick, where he quietly tells them that he can bring Berk peace and promises them that the island will finally end its war against dragons. but only if they bow down to him. The chiefs takes this a joke and laughs at him, humiliating Drago, but Stoick becomes suspicious and cautious. After Drago warns them and walks off two armored dragons quickly descend into the hall and burn everything in sight. Stoick is the only one who manages to escape, of which Drago is unaware.
Years later, Drago is almost finish assembling and preparing his huge army to further his conquest of capturing more dragons, to where they invade the Dragon Den of Valka and the Bewilderbeast. Drago orders his human and dragon army, to keep moving towards the mountain. But proves difficult, when the Riders foil their weapons and traps by suprise, and also encountering the Bewilderbeast, which emerges from the Sanctuary. But Drago, seems to be expecting it, and shows off his own enslaved Bewilderbeast that emerges from the sea crushing anything in its path, which Drago doesn't seem to care. Valka tries to stop him, but she gets pinned down and nearly killed by Drago, luckily Stoick rescues her in time and Drago is suprise to see Stoick alive, the two quickly duel at each other, but he orders his Bewilderbeast to fend them both off. Finally, Drago meets the so called 'Dragon Master' which is Hiccup. Although Hiccup attempts to reason with Drago, Drago ignores his statements and, after his Bewilderbeast defeats Valka's Bewilderbeast, orders his Alpha dragon to kill Hiccup. With the Bewilderbeast now in control of all dragons, in the area, it takes control of Toothless and uses him to attack Hiccup, Toothless initially unable to resist its control. As Drago watches, Toothless ignites his plasma blast to fire at Hiccup, only for the blast to be intercepted by Stoick, killing him instantly as he pushes Hiccup to safety.
Final Invasion of Berk
Drago leaves them to their fate, Hiccup unintentionally making Drago's goal easier when he orders Toothless away in a moment of grief and rage at his father's death. Taking Toothless as his mount- most likely for both the practical reason of wanting the power of the last Night Fury and the symbolic reason of it representing his 'victory'- Drago leads his victorious army towards the invasion of Berk. From there he announces to the villagers that their leader has been killed, and the Bewilderbeast freezes the village with its icy blast.
However, Hiccup and the other riders manage to return to Berk by riding the baby Scuttleclaws outside of the Bewilderbeast's control, with a mocking Drago allowing Hiccup the chance to talk to Toothless. With Hiccup's compassion and bond with Toothless overriding the Alpha's control, Toothless soon returns to himself, throwing Drago off his back and diving down so that Hiccup can resume his usual role as Toothless's rider. Although Drago tries to regain control by ordering the Alpha to kill them, Toothless is able to charge his plasma to protect himself and Hiccup from the Bewilderbeast's ice blast, subsequently challenging the Alpha for control of the other dragons. Drago strongly refuses to stand down and surrender and orders the Alpha to fight back. Toothless, with his repeated plasma blasts weakening the Bewilderbeast's control over the other dragons, they swiftly join Toothless's assault, forcing the Bewilderbeast back and knocking Drago's artificial arm off. The battle concludes when Toothless's final plasma blast breaks the Bewilderbeast's left tusk, forcing it and Drago to retreat to the ocean (It is unknown why Drago's human companions did not rescue him as they weren't revealed to be participating in the invasion of Berk, leaving Drago's fate ambiguous).
Drago is described as a crazed maniac with a human and dragon army that he plans to use to take over Berk, and then the world. Stoick and Valka are supposed to know about Drago. Drago's vast financial power and authority came from keeping the fear of dragons in people and forcing them to join his army. He is steadily gaining control over the world by reinforcing the belief that he alone can control the dragons, so he alone can control people as well. He wants to collect the world's dragons to keep the threat alive and close. He can control those who follow him and get rid of those who won't. The dragon riders make dragons not feared and therefore not a threat. Drago wants to make dragons feared and thinks of dragon riders as a threat. At first he thought Valka was the only one but he learns that there is an entire island full of dragon riders on Berk. Valka sees him as a threat to both humans and dragons.
He is referred to by Stoick as a madman without conscience or mercy, who is a tyrannical leader of his dragon/human army. He is a ruthless, sadistic, callous, megalomanical, fearless, power hungry tyrant who seeks to enslave both dragons and humans and take over the world.
He is also quite abusive, as he has beaten and berated his Bewilderbeast and given his pawn, Eret, a symbolic scar for not bringing enough dragons to him.
Paradoxically, his greatest strength was also his greatest weakness; while his ruthless insanity made it impossible for Hiccup to reason with him as he had originally set out to do, allowing Drago to outmaneouvre and take control of Berk's dragons once Hiccup and the dragon riders unwittingly told Drago of their existence, it also rendered him incapable of understanding Hiccup's bond with Toothless, which allowed Hiccup to regain "control" of his dragon.
He was also quite murderless by making others to do his dirty work, this was shown when his's Bewilderbeast to take control Toothless, forcing him to kill Stoick.
The Haddock Family
The Haddock family poses the gravest threat to Drago's conquest, and he consequently singles them out for destruction before presumably moving on to conquer the known world. Ironically, the only one he ends up killing is the one whom he had not specifically labelled as one of his targets for he didn't knew he had survived - Stoick the Vast, while being defeated by the youngest Haddock who served as his perfect foil in practically every way, especially when came to dragons - Hiccup Horrendous Haddock III.
Hiccup Horrendous Haddock III
After hearing about him from Astrid, Drago seems to acknowledge Hiccup as a rival of sorts, realizing that another dragon-tamer could endanger his monopoly over the dragons. A parallel of sorts to Hiccup, given his own mastery over legions of dragons, Drago is a stark contrast to him as well, using abuse and force to dominate the will of his dragons as opposed to befriending them.
When the two finally face off, Drago, identifying Hiccup upon seeing him ride his Night Fury, Toothless, seems amused that his reputed nemesis was little more than a boy, seemingly dismissing the threat that Hiccup posed. While Hiccup still believed that Drago could be reasoned with, he proves otherwise, stating that strength of will was true power and the only way to control dragons -- an ideal he demonstrates by commanding his grey Bewilderbeast to take control of Toothless, and using him to kill Hiccup. Though this fails due to Stoick's intervention, it succeeds in shaking Hiccup's resolve. Drago then takes Toothless as his mount to further demoralize Hiccup.
During Drago's invasion of Berk, Hiccup continues to surprise him when he and his companions show up on the backs of baby dragons, which the Bewilderbeast could not control, then again when Hiccup's bond with Toothless frees him from the Bewilderbeast's command. The tables are turned on the infuriated Drago when Hiccup's ingenuity and Toothless' own willpower rise up against the great and terrible grey Bewilderbeast, shaming both tyrants and releasing the dragons from their control for good.
Just as Valka despises Drago for his cruel and abusive treatment of dragons, Drago views Valka as his greatest threat prior to finding out about her son, Hiccup, as she was one of the view to stand in his way of complete domination over both man and dragonkind with her safe haven of dragons. While he sees her as an obstacle, he also seems extremely confident that his dragon army and grey Bewilderbeast tyrant would be able to overpower Valka's own dragon legion-- many of which were refugees and not fighters. His confidence proves to be well-founded when his Bewildebeast defeats Valka's white king, and he quickly overpowers Valka in battle, her death only being prevented by the intervention of her husband.
Stoick the Vast
Stoick and the other Viking clan leaders turned down Drago when he came to them with his offer of leadership, laughing him away when he demanded that they bow down to him in exchange for safety from the then-threat of dragons. Seemingly having anticipated this, Drago had several battle-dragons under his control waiting outside, who then proceeded to burn the longhouse to the ground as Drago escaped. Only Stoick survived the encounter, and he sees Drago as a grave threat. During their eventual showdown at the white Bewilderbeast's island, Stoick appears to have the upper hand due to his superior fighting prowess and Drago's disadvantage of his missing arm, and had Stoick not left the fight to come to Valka's rescue, it is likely that he would have defeated the madman then and there. Stoick then intervenes when Drago attempts to kill Hiccup by controlling Toothless, taking the death blow intended for his son. Drago barely even acknowledges Stoick's death when he realizes Hiccup was not the one killed, merely smiling and not even turning to look, suggesting both his sense of superiority over the other man as well as a possible realization that the death of Hiccup's father at the hands of his own dragon would be almost equally damaging.
Eret, Son of Eret
Eret served Drago out of fear, before joining the dragon riders. He gave Eret a symbolic scar for not bringing him enough dragons. Even when he did, he tried to have Eret executed.
Drago found the Bewilderbeast in his early conquests. Drago recognized this as his way to control ALL dragons, whether they like it or not. Drago beat it and even starved it so it was nothing but a huge war machine. He used the Bewilderbeast to kill Valka's Bewilderbeast and then used it to use Toothless to kill Stoick the Vast. In the final invasion, when Toothless challenges the Alpha, Drago looses control and screams "FIGHT!!!" continuously until toothless blows off its tusk, and Drago's mechanical arm. Then, it bowed respectivley to Toothless, and went back into the ocean with Drago on his back, where, without his other arm, very likely could've drowned.
Role in the Series
Since Drago was responsible for the murder of Stoick the Vast, he has become wanted to the High Council, and efforts are being made to capture him, and carry out a horrible punishment. But he is still at large, and nobody was able to find him. He is hopefully still in his world because he has no access to other-worldly travel, and he is probably plotting an act of revenge. But even if he is, he won't be coming back anytime soon.