Catalysm

Just as an FYI, to anyone doing this quest and thinking it maybe bugged because using the flasks on the elementals never seems to work:

The living, corrupted water that flows through their bodies

Sample, then kill. They gotta be alive when you use the flasks on them.

It was misleading because the tooltip for the items said to use it on "the body" of a plagued elemental.

Also take note that it can be used on all elementals in the water, not the purple plagued ones.