Heartseeker

Heartseeker is an enchanted composite long bow +3. It has a +4 thaco bonus and it has the same +2 bonus to damage as a normal composite long bow. The bow weighs 7 lbs and has a speed factor of 4. The combat ability of this bow is that THACO can be increased with an additional 7 points for nine seconds, in which three arrows can be shot from this bow. This item appears in Baldur's Gate II: Shadows of Amn and this item will probably appear in Baldur's Gate II: Enhanced Edition. The weapon is for sale at the Adventurer's Mart in Ribauld's special stock of wares. You can also gain this bow in Windspear Hills dungeon. It's guarded by an adamantine golem.

In-game description
"Heartseeker Legends say that craftsman Pinn O'Reffen fashioned this bow from the heart of a Treant, though how he came to possess such material is unknown. He certainly did not anticipate the enchantment within, magic that makes the archer's aim almost infallible for a short period every day. Pinn claimed his skill was responsible, but it is more likely that some aspect of the soul of the Treant is still within the wood. This bow requires a strength of 18 to use."