To defeat the Burning Legion, you’ll have to first fight its boss – Illidan Stormrage. You will meet him for the first time during the Overwhelming Power quest – one of the first Argus storyline quests. To get to the final fight with Illidan, though, you have to complete Argus quests further. When you get to the Redemption of the Ashtongue quest, Seer Kanai will ask you to defeat Illidan. Fight him at the Black Temple. Bear in mind that his XP is extremely high, so you need a smart strategy. Focus on surviving instead of attacking. However, that’s far from the end of the war. The victory is based on several raids, rather than one boss fight. Follow the Argus campaign until the Death of a Titan quest to finally fight Argus the Unmaker – the world-soul of the planet. The raid has four phases, and you will be helped by Titans. If you work as a team, you will be able to defeat Argus the Unmaker.