Geralt first encountered the hand in Corvo Bianco's wine cellar during his investigation into The Beast of Beauclair, where he saw a bruxa trying to recover it. After killing her, Geralt inspected the hand and saw it still had a heartbeat like it'd never been cut off.
Later, after pursuing the Beast of Beauclair into a lakeside warehouse, Geralt found out that the hand indeed belonged to the Beast, who no longer required it as he had grown a new one.
However, the hand was still important enough to Dettlaff that he had sent the bruxa to recover it for him, possibly to destroy it but Geralt had intervened. Geralt then showed it to Regis who, with some help from the witcher, was able to brew Resonance: a concoction that would let the drinker glimpse into the hand owner's recent past events. Afterwards, Regis properly destroyed the hand as dictated by vampire custom.