Mord'rethar: The Death Gate

Mord'rethar: The Death Gate is the first line of defense used by the Scourge in Icecrown. Behind this gate lies the Valley of Lost Hope.

In order to destroy Icecrown Citadel the gates in Icecrown must first be taken. This is why players of both the Alliance and Horde are sent to sabotage these gates, so the army can conqueror the gates. These quests are either given by Koltira Deathweaver or Thassarian, depending on your faction and involve killing Pustulent Horrors and throwing their spines in plaque-cauldrons to turn the plague into corrosive acid, the latter being better for the troops than the first, according to the death knights, aswell as taking out a powerful zombie, Coprous the Defiled.

After the player has fulfilled these quests, the Death Knight quest giver deems the assault by the big army on the gate to be doable, and sends the player to Aldur'thar: The Desolation Gate.

Triva
Mord means murder in Norwegian, but it is unknown if this was just a coincidence.