Razac

> “Oil from the petals of the Seithr plant, which grows on a small island in the frigid northern seas. In its natural state, the oil is used for preserving pearls—it makes them lustrous and strong. But when specific words are spoken over the oil, along with a blood sacrifice, it gains the property to eat any flesh. That alone wouldn’t make it special—there are plenty of acids that can dissolve sinew and bone—except for the fact that it leaves everything else untouched. You can dip anything into the oil and pull it out unharmed, unless it was once part of an animal or human. This has made it a weapon of choice for torture and assassination. It can be stored in wood, slathered on the point of a spear, or dripped onto sheets so that the next person to touch them will be burned. There are myriad uses for it, limited only by your ingenuity. Any injury caused by it is always slow to heal. It’s rather rare and expensive, especially this converted form.”

Inheritance cycle Book 1, Eragon, Chapter Through a dragon’s eye, pg 243