Water of Brokilon or Waters of Oblivion[1] is a psychotropic, mutagen-containing substance which can turn human girls into dryads. If applied early enough, they are fully transformed, but those who were transformed after being 10 years old will always have their human sweat instead of willow-smelling dryad one.
People with Elder Blood are apparently untouched by the process. When used on witchers it causes severe hallucinations and visions, but afterwards they are in full health.
The goblet
Dryads keep the Water in an ancient goblet from Craag An. It has Elder Runic inscription: Duettaeánn aef cirrán Cáerme Gláeddyv. Yn á esseáth, which apparently means The Sword of Destiny has two edges – you are one of them.