The Quicksilver Bow was crafted for the werewolf hunting ranger Ivar the Night Stalker by his friend Venable the Mad Wizard. It looks like a finely crafted composite bow with silver inlay that ripples and shifts over its surface like the patterns on a cuttlefish. It provides no bonuses to shooting or damage most of the time. When drawn against a werewolf or other target that is vulnerable to silver arrows, some of the quicksilver flows over the head of the arrow, silvering it on the fly.
Ivar found great success using the Quicksilver Bow for over a decade, in an illustrious career of surprising duration for a man so frequently endangered by werewolves, but all things have their costs and he eventually succumbed to heavy metal poisoning. Mad as a hatter for much the same reason, he was imprisoned on a particularly bad day by the city watch of Fellstein. Long and repeated use (on the order of several years) will afflict PCs similarly.
It is rumored that Venable also made a Quicksilver Sheath that treats the longsword drawn from it the same way if it is drawn in the presence of a creature vulnerable to silver weapons.