Oro is the secondary antagonist in the novel Artemis Fowl: The Last Guardian, by Eoin Colfer. He was a fairy soldier who was brought back to life by Opal Koboi to commit genocide against humanity. He eventually was the one to finally kill Opal, the main villain of the Artemis Fowl series.
Oro was the leader of the Berserkers 10,000 years ago. When the faries lost the war for the surface world to the humans, Oro, along with the rest of the Berserkers, were sealed in a Gate, in order to later come back and defeat humanity.
The Berserker Gate
After Opal Koboi escaped from prison, she became a nuclear ,god-like being, and used her power to open the Berserker Gate which was in Fowl Manor. Oro decided to possess the body of Beckett Fowl, who was one of the only humans available.
Opal lied to them and told them she was the leader of the fairies, currenlty at war with humans., She wanted to open the second gate, which would destroy humanity. Although Oro hated humans, he strongly questioned this decision, although he did fight for Opal. He tried to inspire the troups to fight, and was mocked by Myles Fowl, whose body was (incompletely) possessed by the Berserker Gobdaw.
While most of the Berserkers hunted Artemis, Bulter, Holly, and Mulch, Oro guarded Opal while she tried to open the Second Gate. He constantly argued with her, but she was able to control his actions since she opened the Berserker gate.
Opal was tricked by Artemis into letting her clone close the Second Gate. This freed Oro from Opal's bond. Opal commanded Oro to kill the clone. Oro protested that he did not want to kill a fairy, and Opal declared that she had killed many fairies, and would kill countless more to achieve her goals. When Oro heard this, he killed Opal.
After Oro left Beckett's body, some of his magic remained there. Thanks to him, Beckett had the Gift of Tongues, and was able to talk to animals and trolls.