In Undercity, an Undercity Guardian yelled out ""Scourge in the Sewers! We need help!"" (They can also yell out other locations - just search the location of the attack and you might find a putrid horror with a bunch of flameshockers )

I went to the sewer entrance (49.2, 29.1) into Undercity on the outer ring where the ramp goes down from the sewers and there was a Putrid Horror with 5-6 Flameshockers. When we killed the Putrid Horror, the rune dropped on the ground and offered a quest. Thus, the quest will be available for whomever happens by it.

When the mobs are killed Sylvanas yells ""The Scourge attack against my court has been eliminated. You may go about your business.""

For fun it's suggested that you then yell ""YOU SHALL NOT PASS!"" for pure amusement, or not. it's up to you. :)

But, the Putrid Horror's death is what caused the rune to drop on the ground that offers the quest for anyone around. I'm not sure if and where this occurs in other cities.