Black oil

This black liquid only affects mammals. It causes black coloring of the pupils of his victims, but allows to better withstand the pain. Some diplomats or spies use it when they fear being tortured.
A creature subjected to this poison must make a DC 20 Constitution saving throw. In case of failure, the creature sees only as a human in darkness but also gets resistance to bludgeoning damage, all of that during 1d4 hours.