Biter

First appeared in Siege of Dragonspear, this item can be imported into BG2EE, it's later found at Waukeen's Promenade Shop.

In-game description
"Biter +2 This dart gleams like a streak of light when flying through the air. When it hits its target, it releases a tiny acid droplet before soaring back to the hand that threw it. Sometimes the acid manages to burn a hole in the target's armor."