The Siege of Dalaran is Chapter 7 of the Undead campaign, Path of the Damned.
Dalaran, a Human nation known for excellent magicians, has been the chosen site for Archimonde's arrival to Azeroth. After Antonidas heard Arthas is coming to them, he prepared special auras that kills Undead.
Unbeknownst to Dalaran, the beasts, animals and ogres kept for experiments get captured by the Undead, and their immunity to the Aura allows the Undead army to push through Dalaran's defences, killing the Archmages who cast the auras until they find the last Archmage, Antonidas, and end his defense.
- 5 Sheep
As Arthas ordered the Kirin Tor's surrender, Antonidas came to greet him.
- Arthas: Wizards of the Kirin Tor! I am Arthas, first of the Lich King's death knights! I demand that you open your gates and surrender to the might of the Scourge!
- Antonidas: Greetings, Prince Arthas. How fares your noble father?
- Arthas: Lord Antonidas. There's no need to be snide.
- Antonidas: We've prepared for your coming, Arthas. My brethren and I have erected auras that will destroy any undead that pass through them!
- Arthas: Your petty magics will not stop me, Antonidas.
- Antonidas Pull your troups back, or we will be forced to unleash our full powers against you! Make your choice, death knight. (Antonidas teleports away)
- Kel'Thuzad: I sense that three separate wizards are maintaining these auras. If you find and kill them, the auras will disperse.
Arthas entered the city with his army. They killed the first wizard.
After which they found cages.
- Kel'Thuzad: The Kirin Tor keeps these creatures caged for studying. If they were to be released, they would cause our enemies a great deal of pain.
As they marched further, they encountered the base of the Mage's guild. After the base was destroyed they encountered the second Archmage.
As the assault continued Arthas found a vault.
- Kel'Thuzad: The wizards keep many enchanted items within that vault. Shatter it, and we can use their own weapons against them.
And then attacked the base of the Sorcerer's league. As they moved on they reached a crossroads: To the north was Antonidas, and to the west was The Kirin Tor's base. Arthas destroys both.
- Antonidas: It pains me to even look at you, Arthas.
- Arthas: I'll be happy to end your torment old man. I told you that your magics could not stop me. (Arthas kills Antonidas)
Then Arthas and Kel'Thuzad retrieve the Spellbook of Medivh
- Arthas: The spellbook is all yours, lich. Let's take it and leave before the wizards amass for their final attack.
- Kel'Thuzad: Indeed. I will begin summoning Lord Archimonde at sunset.
- Non-Undead units cannot be damage by the aura, and you can use this to your advantage by researching Banshee Master Training and training large numbers of Banshees, and then wait for the Dalaranian troops to attack. Use Possession on the enemy troops, and your forces can attack the enemy bases without suffering from the aura.