This quest will trigger once you’ve completed A Posh Recepetion and either Six Feet Under for Siegried or Echoes of Yesterday for Yaevinn. When you enter the Temple Quarter or the Trade Quarter a young boy will approach to tell you that the Bank (#1) is being robbed.
Quickly make your way to the bank to find Velerad and the City Guards standing outside. The Scoia’tael have taken control of the bank and have taken hostages. Speak with Velerad to work out what you want to do. You have a few options including entering the Bank via the Warehouse (#2) next door, the Town Hall (#3), waiting for Siegfried to show up (if you’re on good terms with the Order) or entering the bank to rescue Seigfreid (If you’re on good terms with the Scoia’tael.
If you go through the Warehouse you’ll need to kill a few Fleders and then go up to the second floor. After killing more Fleders go through the door to the upper level of the Town Hall. From there you can take the stairs down to the Bank.
Once you find Yaevinn he may ask you to join the Scoia’tael. If you refuse you’ll have to fight the Scoia’tael and Yaevinn will flee down into the Sewers. Run back to the front of the bank and tell Velerad that he escaped to complete the quest. If you join the Scoia’tael you’ll make your escape with Yaevinn and fight off a number of large Kikimores along the way. He’ll be hiding out at Golan Vivaldi’s house from now on.