In Terry Goodkind's series, the Sword of Truth, the Sliph was a woman made from a liquid silvery thing resembling quicksilver. The Sliph was used to travel almost instantaneously from place to place in the Old and New Worlds, the trip required you to breath her in, instead of oxygen, or else you would die. This gave you a euphoric sensation for the duration of the trip, causing you to not want to breath back in oxygen once out of her.

Richard discovered the Sliph in Blood of the Fold in the Wizard's Keep (a castle built by the war wizards of ancient times). Later in one of the books (can't remember, I get the books confused from time to time because I practically inhaled one after the other) the origins of the sliph are revealed, all I'll say is she came from the great war between the Old and New Worlds, like the mriswith and gars, I don't to give out any plot spoilers or anything.