The ability in question is mechanically similar to the Tsunami spell, which only affects creatures.
However, the Leviathan is a siege monster, and its description states:
Siege Monster. The leviathan deals double damage to objects and structures (included in Tidal Wave).
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.
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 rules for siege monster 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 this 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?