Brinesoul Feat 1LineageUndineSource Ancestry Guide pg. 118
Your elemental ancestor's legacy asserts itself as brackish water, like the seawater of the ocean. You gain the Salt Wound reaction.
Salt Wound 
(
undine)
Frequency once per day;
Trigger A creature that has blood and is not at its maximum Hit Points hits you with a melee Strike;
Effect You channel salt and brine from your blood into the creature's wounds. The creature must attempt a Fortitude save using your class DC or spell DC, whichever is higher. On a failure, the creature takes 1d6
persistent acid damage and is
sickened 1 by the pain. On a critical failure, it instead takes 2d6 persistent acid damage and is sickened 2.
Traits
Lineage: A feat with this trait indicates a character's descendance from a particular type of creature. You can have only one lineage feat. You can select a lineage feat only at 1st level, and you can't retrain into or out of these feats.
Undine: Undines are planar scions descended from marids.