The summer's kiss is a rare and costly, but extremely deadly poison. It is clear and tasteless, and thus leaves no trace that might alert the target. The poison is obtained from the summer ivy plant which is located in the Borderlands.
When someone is exposed to the poison, they become frenzied and attack anything, or anyone, in sight. After a short while their organs fail to function and they die a slow and painful death.
In the books
In the A Song of Ice and Fire novels, the summer's kiss are apparently made in the Borderlands. It causes insanity and kills very quickly. Not only is it difficult to create the poison, House Blackgard has also strictly forbidden the manufacture and use of the summer's kiss. Nonetheless, the summer's kiss is still a famous product in the Borderlands and is used by the Bordermen not only for battle, but also for trading purposes.