The Ambush

The Ambush appears if Gorion's Ward was able to prove their innocence to Schael Corwin in the Abandoned Sewers & Caverns. In a continuation of the cutscene that started from the Sewers Exit, Imoen will say that she did not release Gorion's Ward from the Flaming Fist HQ, Basement. She will then ask if the party feels off. Shadow Thieves will appear, and Imoen, Minsc, Dynaheir, Khalid, and Jaheira will say a few lines before they and Gorion's Ward fight them and fall unconscious. A Shadow Thief will tell his men to load them onto the cart quickly as their client wants his prize.
This scene ends Baldur's Gate: Siege of Dragonspear and leads into Baldur's Gate II: Shadows of Amn.