Drow Crossbow of Speed (Drow Crossbow of Speed +3 in-game) is an enchanted light crossbow +3 in terms of enchantment. Like all adamantine items, this will turn into Adamantine Dust when it's taken out of the Underdark.
This weapon is nearly the same as the Light Crossbow of Speed, but since the mentioned crossbow is a +1 weapon, the drow variant weapon with +3 bonus to damage and -3 THAC0 with a speed factor of 1, is a lot better. But the +1 crossbow isn't made of adamantine and therefore can be used anywhere.
The Drow Assassin and Drow War Scout can drop this weapon in 
Baldur's Gate:
Siege of Dragonspear (2016)
This icon indicates content from the Siege of Dragonspear campaign of the Baldur's Gate: Enhanced Edition., and Drow and Drow Warrior (Underdark) can also drop this weapon in 
Shadows of Amn
Classic & Enhanced
This icon indicates content from the Shadows of Amn campaign of both the original and the enhanced editions. and 
Baldur's Gate II: Enhanced Edition – Shadows of Amn (2013)
This icon indicates content from the Shadows of Amn campaign of the Baldur's Gate II: Enhanced Edition.. The Drow Warriors in Sendai's Enclave in 
Throne of Bhaal
Classic & Enhanced
This icon indicates content from the Throne of Bhaal campaign of both the original and the enhanced editions. also employ these weapons. Most drow fire Drow Bolt, Drow Bolt of Sleep and or Drow Bolt of Stunning from this crossbow. A typical warrior will have a quiver with two kinds of ammunition available.
Description[]
| “ | A crossbow is a bow mounted crosswise on a wooden or metal shaft, the latter called a tiller. The bow is usually made of ash or yew. The crossbow fires a quarrel (also called a bolt).
This item, like most drow equipment, is created using adamantine, an alloy of adamantite that quickly turns to dust if removed from the Underdark. |
” |