Encounter Feature: Flood Wall

 This is just a quick look at a possible encounter feature I came up with. I don't think it's quite accurate to call it a trap, per se, as it does not act of its own accord; it's really more of a weapon built into the environment, in this case a weapon which endangers the user almost as much as their targets. I'll be using it in my first encounter post this Sunday!

A Flood Wall is any flat surface which is used to hold back a large amount of liquid or fine sand. In this case, the Flood Wall is used primarily as something like a bulkhead in a ship or a window in an underwater city.

The flood wall has 50 hit points, AC 8, and a damage threshold of 20, much like other large objects like boats; a DC 20 Strength (Athletics) check can also be used to instantly break the flood wall if one stands right next to it. When the flood wall is broken, water immediately begins to flood the room, forcing each creature in a 30 foot line in front of any 5 foot space of window to make a DC 13 Strength saving throw, taking 4d10 bludgeoning damage and being knocked prone on a failed save. On a successful saving throw, the enemy takes half damage and is not knocked prone. At a depth which might be considered dangerous to submarines, such as a trench, consider changing the Strength DC to 17 and the damage to 10d10, as well as pushing the target 20 feet regardless of whether they succeed on their save. This attack persists until the room is filled with water, so it is inadvisable to attempt to bypass the torrent.

Once a flood wall has been broken, the room will begin to fill with water. Each round, a number of square feet equal to the square footage of the hole in the wall (likely the entire window frame) multiplied by 5 is filled with water. As such, even a single 1 foot wide port hole can completely fill a 50 square foot room with water in 1 minute. An extravagant 25 square foot window could fill a 1250 square foot room if the Barbarian is stupid or audacious enough to smash it. Water which is 2 and a half feet deep (or less than 5 feet, to simplify) is considered Difficult Terrain to Medium creatures, and requires Small creatures to swim. 5 foot deep water is considered Difficult Terrain to Large creatures, and requires smaller creatures to swim.

This feature is great for underwater areas, and can also be converted to work with sand in a desert temple or ancient tomb.