The Dream Eater

Draco Malfoy has found a most interesting potion. Moste Potente Potions lists only the legal ones, the ones everyone tries. The ones they end up learning about in class at one point or other. The one he found is listed in a book with the title page ripped off and stuffed into the back of his father's library, hidden behind a stack of books on necromancy. Half of it is written in a language Draco can't read, but the recipe for the potion is written in English.