Black lotus extract

Certainly, the most dangerous poison in the world. Fortunately, it is also the most difficult to find because of the extreme rarity of this plant and the exceptional care to give to the preparation. No one escapes and death is almost instantaneous. It is an odorless, colorless and virtually invisible liquid.
A creature subjected to this poison must make a DC 20 Constitution saving throw, taking 55 (10d10) poison damage on a failed save, or half as much damage on a successful one.