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The Leviathan's Tidal Wave action says (MToF, p. 198):

Tidal Wave (Recharge 6). While submerged, the leviathan magically creates a wall of water centered on itself. The wall is up 250 feet long, up to 250 feet high, and up to 50 feet thick.

When the wall appears, all other creatures within its area must each make a DC 24 Strength saving throw. A creature takes 33 (6d10) bludgeoning damage on failed save, or half as much damage on a successful one.

At the start of each of the leviathan's turns after the wall appears, the wall, a long with any other creatures in it, moves 50 feet away from the leviathan. Any Huge or smaller creature inside the wall or whose space the wall enters when it moves must succeed on a DC 24 Strength saving throw or take 27 (5d10) bludgeoning damage. A creature takes this damage no more than once on a turn. At the end of each turn the wall moves, the wall's height is reduced by 50 feet, and the damage creatures take from the wall on subsequent rounds is reduced by 1d10. When the wall reaches 0 feet in height, the effect ends.

A creature caught in the wall can move by swimming. Because of the force of the wave, though, the creature must make a successful DC 24 Strength (Athletics) check to swim at all during that turn.

Its Tidal Wave action is mechanically similar to the tsunami spell, which only affects creatures. However, the Leviathan has the Siege Monster trait:

Siege Monster. The leviathan deals double damage to objects and structures (included in Tidal Wave).

I have some questions about how the Leviathan’s Tidal Wave interacts with objects:

  1. Does the wave move through walls?

    What happens when the tidal wave encounters a wall, or any object whose size is significant compared to the wave? Does it clip through the object, like bad Roblox physics, possibly killing everyone inside?

    I have a proposition: the wave damages the object, and if the damage is enough to collapse the wall, then it moves through the object. Perhaps if it is a ship, then you could describe it as the ship being capsized. However, if the wave isn’t enough to destroy the object, then the wave curves around it. Forget conservation of mass, if the object is bigger than the wave, then perhaps it could ghost through it (move through it but without existing inside it) and reappear behind it, if the object is small enough.

  2. What happens if a creature, caught by the wave, hits an object?

    Let’s say that you hit an object big enough not to be destroyed by the damage, what happens then? Surely you wouldn’t phase through the object to continue following the wave, nor would you curve around it to do so (it might not always even be possible).

  3. How much damage does Tidal Wave do to objects?

    The Siege Monster trait would indicate that the damage (6d10, 5d10...) is doubled for an object. So 2×(6d10) ...?

This is about a CR20 monster whose description states that it destroys coastal settlements. So surely it can damage buildings, but in-game, how does it work?

Perhaps its Water Form trait would help:

Water Form. The leviathan can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Surely a wave is more so water than a monster? So perhaps the wave can also enter a closed space, so long as there is at least 1 inch of space? Does this make sense?

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    \$\begingroup\$ You asked three questions in one. That's usually a bad form here on Stack Exchange, but in this case these are interconnected, so maybe that's OK? \$\endgroup\$ – Mołot May 16 at 17:01
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    \$\begingroup\$ That was the point, the questions being interconnected, I thought that the answers to one of them could help figure out the others. \$\endgroup\$ – Attonwizard May 16 at 17:17
  • \$\begingroup\$ Most of the time it is better to ask them in series, but the line is blurry. \$\endgroup\$ – Mołot May 16 at 17:21
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    \$\begingroup\$ As you know, the answers to your questions are not in the rules. The chances of someone having already house-ruled and play tested this exact scenario is slim. What would be an acceptable answer for you? Are you the DM or a player? Is there something stopping you from just applying the given creature damage numbers to objects? \$\endgroup\$ – user-63873687 May 17 at 4:02
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    \$\begingroup\$ Well I wasn’t sure that there isn’t an explanation in the rules, that’s why I was asking. I can always split the question into three parts. I’m a DM, hoping to use this. Nothing’s stopping me, I just feel reassured when I use abilities as intended by the authors. \$\endgroup\$ – Attonwizard May 17 at 9:27
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  1. RAW, the wave does not interact with objects, although it should from a perspective of verisimilitude. Considering common knowledge of real-world behavior of water, it seems like there should be three cases.

    a) Object is very small (compared to the wave): Applying the rules for creatures seems reasonable in this case.

    b) Object is not small but neither the same size as the wave. The wave should deal damage to the object and potentially destroy it. If the object is not destroyed the wave will still wash over it but lose some height / force. This can be done using the rules from the trait, i.e. the wave could lose two turns worth of height in one turn in a specific direction when washing over a wall.

    c) The object is similar in size or larger than the wave. The wave might deal damage (e.g. if the object is a wall that is high but not very thick) or it might not) against a mountain. If the object is dealt damage, it might be destroyed (vide infra). If the object (e.g. wall) resists the wave but has holes and / or is a bit smaller than the wave, it acts as wavebreaker, i.e. the destructive force is removed, but some water will get on the other side.

    (A note on Water Form: This is the expected physical behavior of water. It probably exists because the leviathan is considered a creature and the physical behavior is not what we expect from creatures).

  2. There are monsters in the monster manual that have rules for hitting creatures against each other or against objects (Water Elemental, Kraken, Otyugh). These can be used as inspiration for situations where someone is washed against a wall. It is important to make note of the low velocity of the wave though (as indicated by Charlie Holmes). Realistically, this is roughly equivalent to walking against a door. Therefore, creatures are rather drowned then crushed.

  3. Has been answered by Charlie Holmes. If you decide to apply the damage rules to objects, Siege Monster should also apply (both RAW and from a perspective of verisimilitude).

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Some thoughts on how I would handle it.

I'm going to try to answer this with how I would rule a hypothetical ship getting hit.

Key point: 50ft per round is 5 miles per hour. The wave is not moving fast, it is tumultuous and huge but it is slow.

So the Tidal Wave enters the space the ship is in and I would say the ship rolls a strength saving throw. There are stats for ships in a UA called Of Ships and the Sea and I would use the strength from that for the ship's modifier and then I would say that it did do double damage as a Siege Monster.

Creatures on deck have to make the strength saving throw with +5 if they have hold of something to stay with the ship. If PCs are on deck I would let them use a reaction to try and grab something within 5ft if they have a free hand, with a Dex save of 15. I would let them drop something as they did this but I would explicitly tell them that it will probably get washed overboard.

Creatures in the body of the ship don't get hit by the wave but I would have them roll a Dex save for the initial impact. If they fail the damage would be 1d6 at low levels 2d6 at higher levels. Water starts pouring in from the top of the ship, through gaps, and through windows where the ship is in the wave. I would say that the ship starts to take on water and maybe loses 5 ft of movement after 3 rounds and 10ft if the ship is in the wave for the full 5.

The ship would have to roll strength for movement and the movement would be relative to the wave so the boat had to sail through the 50 ft thickness of the wave to get out.

So to directly answer the points:

  1. I would say it didn't clip but that it would still be like a wave on top, around and coming through holes and whatnot. If it is smaller vertically than the object then it does it's damage and if it's still standing it is redirected as you see fit.
  2. The wave is only moving 5 miles per hour so I would not include more than 2d6 damage for being hit into something and I would give them a chance to try to stay with the object. Probably a 19DC Strength save to brace themselves against it as the wave is redirected and tries to take the creature with it.
  3. Yes double the damage to objects, ships, wall, etc.

Water form is for the Leviathan not it's wave.

I think it is likely that many creatures that were on deck are going to find themselves in the water afterwards.

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    \$\begingroup\$ Can you support why you would handle it this way or, even better, how your method of handling it worked at your table? \$\endgroup\$ – NautArch May 19 at 15:06
  • \$\begingroup\$ I've used elements from this for a storm that lead to a shipwreck but mostly I am just attempting to combine the rules available with what seems realistic to me. If I can't imagine it happening I don't tend to make it happen that way regardless of RAW. For example I tweak sentinel in my games so that large and smaller lose movement, huge it gets halved and gargantuan ignore the movement penalty. \$\endgroup\$ – Charlie Holmes May 19 at 15:11

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