Mainly for Rogues but other classes brave enough can try.
Became famous/infamous on your server by being a city assassin
This activity is only possible recently with Warmode and only SOME people will have pvp enabled after taking the portal.
Wait for those people
How:
Orgrimmar: Cleft of Shadows
Stormwind: Mage Tower
Catch players who are entering a portal to the city
You need the item Net-o-Matic 5000 on Orgrimmar to dismount players from their flying mounts
If you are a rogue, spam "Sap" on the place where they are about the spawn and then use the item to make them dismount.
Escape Routes
Orgrimmar: You can run to the low level instance nearby
Stormwind: As a rogue you want to exit the mage tower with sprint and Vanish and run to the basement of this building immideatly to the right.
Dont let them know you are running to this building. Vanish WAY before and RUN to the basement.
Here is the place where they spawn in Orgrimmar. In Stormwind they spawn literally inside mage tower (there is a semi-mini portal there)