Twin Sting is a soulbound crossbow in Pillars of Eternity, added in The White March - Part II expansion. It can only be used by a Cipher, Ranger or Rogue. It can shoot twice before it has to be reloaded.
As with other soulbound weapons, the in-game description of Twin Sting is only gradually revealed. The description starts as a single paragraph, and more are added each time the weapon is upgraded.
Twin Sting is a crossbow with an unusual design: it is built to fire two bolts before requiring a reload. Few are the practical uses of such a weapon that would be worth the trade-off of the extra weight. This item, however, was built to fulfill a very particular need.
Local chapters of the Guild of the Poison Scale, a group of fanatical Woedican assassins who believe the world is safest when no single entity wields too much power, spread from Aedyr to Dyrwood in the years before the Dyrwoodan revolution, and have continued to operate there since.
Darden Guthroc failed his first assignment as a member of the New Heomar branch of the Poison Scale. It was not for lack of skill, but rather lack of contingency planning, that his attack was spoiled. He tracked an ascendant young noble to his lover's abode, and waited all night from a rooftop across the road, his crossbow trained on the door.
When the noble emerged the next morning, Guthroc's poisoned bolt struck him just above the heart. However, the noble was a man of some girth, and the dose of poison was too sparing. The man ran through the streets of New Heomar screaming and waking up half the city, including an apothecary who quickly administered an antidote before the poison had taken full effect.
Such was the embarrassment to the Poison Scale that Guthroc was nearly cast out. But the leadership knew that Guthroc's promise was great. They had built for him a custom crossbow capable of firing two bolts with a single shot, that he might never make the same mistake twice.