- "The tears of Lys, they call it. A rare and costly thing, as clear and tasteless as water. It leaves no trace."
The tears of Lys is a rare and and costly, but extremely deadly poison. It is clear and tasteless, and thus leaves no trace that might alert the target.
In the books
In the A Song of Ice and Fire novels, the tears of Lys are apparently made in the Free City of Lys. It causes extreme sickness in the bowels and belly, and kills very quickly. Not only is it difficult for the intended target to detect, its effects are subtle enough that they are easily mistaken for a very sudden and severe fever. Even upon examination of the corpse, it is difficult to discern if someone has been killed using tears of Lys.
- The Tears of Lys at A Wiki of Ice and Fire (MAJOR spoilers from the books)