D&D 5th: Demon Tower of Madness
Scroll, Airy Water
Property [Consumable]: If the spell written on the scroll appears on your class’s spell list, you can use an action to cast it without having to meet any of the spell’s requirements.
This spell turns normal water (or watery solutions) into a frothy substance that is as breathable as air. Both air-breathing and water-breathing creatures within the area can breathe normally. Moving through airy water is easier than moving through normal water. Swimmers make a DC 10 Swim check once per round; they swim at their normal speed on a success, or half their normal speed on a failure.
Creatures with a swim speed can simply use their swim speed without penalty.
Creatures can instead use their land speed to move along the bottom at their normal speed.
Creatures walking on the bottom do not pay any movement penalty for walking on sand, stone, or pebble seafloor and only pay 2 squares of movement per square of muck or mud entered.
Finally, airy water negates underwater combat penalties for all melee attacks, although ranged attacks still take the normal underwater penalty.
The spell does not filter or purify solid particles, so casting airy water on very turbid liquids, for example, would produce an area filled with dust or smoke (and impose similar penalties).