Doppelganger

Doppelgangers are insidious shapeshifters that often use their abilities to infiltrate powerful groups or organizations for various purposes. A stronger version, the greater doppelganger, is also capable of casting spells.

In Baldur's Gate they have been used by the Iron Throne (especially Sarevok) to infiltrate and undermine the Seven Suns trading coster and the Merchants' League in order to increase the Throne's own profits and control of the region.

Investigating the Merchants' League Estate
Aldeth Sashenstar (or his brother) is convinced that something very strange is going on in the Merchants' League, and he asks Gorion's Ward and their party for help.