SPOILERS
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-You have to talk both to the Female Captain (shes in Beregost, in the tavern located in the south-east of the town) and to Scar of the flaming fist in the bridge.
Also, the entrance to baldurs gate in the bridge isnt very clear, you have to go west till you meet Quayle and then go past some walls and click on the western edge of the map to enter the city
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"I miss the Angel... and I mean that I REALLY miss her" -Ertai, Captive of the Blinding Angel