Purgewater Canister Harpoons are a type of ammunition in Horizon Forbidden West compatible with only one Ropecaster. While it deals very low Impact damage when attaching to a target, the canister can be set off by Purgewater arrows, triggering an explosion that deals high Purgewater damage and very high Purgewater buildup.
These can be used to inflict the Drenched state on enemies, which temporarily removes their ability to inflict elemental damage and makes them susceptible to all elemental buildup, even if they are normally resistant to an element. Effective against enemies with purgewater vulnerability.