Siege of Dalaran

The Siege of Dalaran was a battle that took place during the Third War. After raising the necromancer Kel'thuzad as a lich in the invasion of Quel'thalas, Arthas Menethil, champion of the Lich King and the Scourge took his forces to the great magical city of Dalaran, where the lich needed to acquire the Book of Medivh, one of the spellbooks belonging to the former Guardian in order to summon Archimonde and the Burning Legion to Azeroth. However, the mages had already prepared for the invasion, and had evacuated its citizens in advance. Furthermore, they erected protective fields over the city that would damage and undead that passed through them and blocked all teleportations into the city.

Thanks to Kel'thuzad, Menethil knew that the fields were channeled by three mages and that it would dissipate if they were slain. Despite the presence of the anti-undead field, the Death Knight sacrificed many of his troops to enter the city and fought his way through the Dalaranian forces, slaying the first two archmages holding the auras and rampaging through the Mages' Guild and the Sorcerers' League in the process, even releasing some of their captured creatures to aid in the fight. After a fierce battle, the Scourge reached the final archmage holding the aura, who was none other than Antonidas himself. Though his defenders fought intensely, Menethil murdered the elderly archmage and dispersed the auras over the city, retrieving the Book of Medivh.

Summoning the Legion

 * "Tremble, mortals, and despair! Doom has come to this world!"
 * — Archimonde arrives in Azeroth

After stealing the spellbook, Menethil and Kel'thuzad retreated to their base camp where they prepared to summon Archimonde. However, the lich warned him that the remaining mages will be able to sense the power of his spell, and will be determined to disrupt the ritual. Kel'thuzad began the summoning ritual, leaving Menethil to lead the Scourge's defenses.

The mages launched a massive assault on the Scourge base, but they were ready. Early in the battle, the Death Knight's scouts reported their discovery of a stockpile of Goblin land mines they later used to reinforce their defenses, and as the summoning progressed the barrier between Azeroth and the Twisting Nether weakened, allowing the Legion to send their own forces to aid the undead; Infernals rained from the burning skies and Hellhounds bolstered their ranks. The combined forces of the Scourge and the Legion held the forces of Dalaran at bay long enough for the summoning to end, and Archimonde entered the world.

At the mere sight of the Defiler, the remaining Dalaranian forces fled to the city. Archimonde relieved the Lich King of the command of the Scourge and turned them over to the Dreadlords, and he proceeded to destroy Dalaran with a single powerful spell.