The Sleeper

The Sleeper is an enchanted morning star +2. It has the same properties as the Morning Star +2, but this weapon can make any human or demi-human fall asleep for 18 seconds, if they fail their saving throw against poison with +4 bonus when hit by this weapon Elves are immune to this effect. This item appears in Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition.

It can be bought from Bernard in the Copper Coronet, but only after some quest related to the tavern are complete.

In-game description
"Morningstar + 2: The Sleeper This belonged to Ssitalc, an uncharacteristically evil elf known as the Slaver of the Sword Coast. Until his sudden death several years ago, Ssitalc commanded a large force of human, dwarf and gnomish brigands, using the Sleeper to keep them in line. It has a chance to incapacitate any human, dwarf, gnome, or halfling by inducing deep slumber, though elves are conveniently immune."