In Baldur's Gate, it can be gained from the brother of a hunter in the Cloakwood if the hunter is killed in a subquest he is involved in. The brother, Dabron Sashenstar, who is in the city of Baldur's Gate, will try to avenge his death if he meets with the party and will drop the weapon when he is killed. This crossbow and the Light Crossbow of Speed are the best crossbows; the scythe has a better rate of fire, but is not as accurate and adds less damage to bolts.
In Baldur's Gate II, it is dropped by one of the Cult of the Unseeing Eye guards after you have killed their Beholder God.
|“||Heavy crossbow of accuracy: 'The Guide'. A favored siege weapon of King Shastir Longeve, crossbows of this type were designed for the systematic removal of defensive forces thought safe behind the battlements of castles. Scant few of these weapons survive to this day, mainly due to the eventual annihilation of the king's forces by a coalition of subjugated lands.||”|