An elemental composed mostly of water that is exposed to a pinch of the dust must make a DC 13 Fortitude Roll, taking 10d6 necrotic damage on a failed save, or half as much damage on a successful one.
You sprinkle the dust onto the water elemental; its liquid form shudders as sections evaporate into vapor, its once-fluid body shrinking and distorting under the dust's dehydrating power.