So yeah, spoilers, so
North of the Abandoned Village, jump across the broken bridge, then head West. Further east of Waukeen's Rest (the burning villa) you should approach an intact wooden bridge. A red dragon should fly overhead (possibly applying temporary Frightening Presence). A tiefling should warn you about the Githyanki below, and a cutscene will basically tell you in no amount of uncertainty not to mess with the Red Dragon. Anyhow, head down the ladder on the left of the bridge.

Super hints - be prepared for Lae'Zal to drop out of your party to approach the patrol. Its also a good idea to leave one party member on the bridge high ground in case a fight breaks out (and choose someone with decent strength or Thunderwave in the event you have to shove). If you interrogated Zorru, you'd get a square quest marker on this location.