Zor'be the Bloodletter at the Argent Tournament Grounds wants you to kill 15 members of the Cult of the Damned. You may kill cultists in any part of Icecrown.
While we all play at mock combat, it will do us good to remember that the real enemy surrounds us. The Argent Crusade wants to use competition to find the best fighters, but we can tell that by who returns from battle. Let's show the paladins that we are willing to take battle to the Cult of the Damned, even while making preparations for the tournament.
Go forth into Icecrown and slay any of the cultists you encounter. They have a strong presence around Aldur'thar and Deathspeaker's Watch.