Assassin's spear

An Assassin's Spear is a +1 magical shortspear that conveys this magical enhancement bonus when used in melee combat, but if thrown, grants the thrower invisibility. The enhancement bonus does not function for the duration of the invisibility spell, so it strikes as a normal shortspear. But once the invisibility spell is broken or wears off, the Assassin's Spear regains its magical bonus until the next time it is thrown. An Assassin's Spear is worth around 8000 gold pieces.

Realmslore
The spear was made by a master assassin in the city of Amn who had become bored with all the methods of killing people he had tried. He wanted to be able to kill people in a large crowd in a spectacular fashion and yet still escape undetected. He created five weapons in the form of spears, since these were often carried by city guards, but told no one of their powers. These five items are now scattered across Faerûn.