Speak with Warden Thelwater in the Stockade.
VanCleef and I were members of the Stonemasons Guild. Our main project was to rebuild Stormwind after the War.
When we had finished our duties, we were cheated. The nobles refused to pay us for our work. Some of the more senior of the Stonemasons were offered governmental jobs, but VanCleef refused it out of loyalty to all the Stonemasons. He led a riot and left the city, swearing revenge.
His lieutenant, Bazil Thredd, might know more about VanCleef's plans. He's being held prisoner in the Stockade.