Thanks for this Ivan - enlightening as always. Actually, going into this detail raises more complications I'd not even thought of...
The new and confusing thing with Tsunami is the ability to push objects that are three or four zones away, such as a Mage in B1 pushing objects in C3. This is neither truly diagonal nor orthogonal, but rather at an angle in between. The rules don't go over this possibility in any detail, and the final ruling may end up contradicting me, but based purely on what's currently in the rulebook, this is how I would handle the situation.
I would do like Donovan suggests. Draw a line from the center of the Mages zone, through the center of the target zone, and then continue that line until you hit another zone. Whatever zone you hit is where you MUST push to. This is what best fits the description of moving them "one zone away in the opposite direction" if you ask me.
That's a reasonable ruling; I would have ruled that where it's not in a straight line you could push to either zone, so a 2,1 or 3,1 diagonal is handled the same way as a 1,1. That said, thinking about it, your way is more consistent with LOS past walls I believe...
Next, we need to reiterate that the source of the spell is the caster. Not the zone the caster was in when they cast the spell. The source is the caster. If the caster moves, then the source of the spell/attack/push moves as well and you will start drawing your lines from the new zone that the caster is in.
...And
the source of the Push effect is the caster, not the spell. These two small but very significant rules that open a whole can of worms... and more than just the push issue which I'd spotted.
Having a Tsunami travel one direction, but resolve pushes perpendicular to it (or even reversed from it in the first zone) is weird and non-intuitive. It's well-supported in rules, but I think this card should be ruled differently.
Do you need Line of Sight?
Pushing does not actually require line of sight. It is certainly possible that the caster of Tsunami may be moved to a zone in which they no longer have line of sight to the target zone that they were in the process of attacking. This will not prevent them from resolving the attacks in that zone in any way shape or form (other than perhaps changing the direction of the Pushes).
Again, Tsunami hitting a zone, but pushing creatures affected before the mage one way, and after the mage another way falls into the 'weird & unintuitive' bucket to me.
However, you do have to have line of sight to a zone in order to target it AND initiating a zone attack always requires you to target the zone you're attacking. You don't have to target the individual objects in that zone, but you do have to target the zone itself. Thus, if line of sight is ever blocked from the caster to a zone that Tsunami wishes to initiate an attack in, the attack will fail and be cancelled. Since the entire zone attack failed to resolve in this situation, I do not believe you'd be allowed to repeat it in another zone even if you somehow have line of sight to that new zone. This could be another area where I am incorrect though.
So... The following situation; where the caster is in A2, and the Tsunami first targets A2 travelling 'eastwards' along row 2, and an LOS-blocking wall exists between B1 and B2:
A1
B1 C1 D1
A2 B2 C2 D2
A3 B3 C3 D3
Scenario 1: the Tsunami can throw the caster out of the Tsunami to A3, and other creatures forward to B2. Then the Tsunami attempts to push everything remaining forward into C2, then attacks C2, and pushes things into D2, before finally attacking D2, and then probably throwing things into the Arena wall. This is all how one would expect things to work.
Scenario 2: the Tsunami instead throw the caster out of the Tsunami to A1, and other creatures forward to B2. Then the Tsunami pushes those remaining forward into C2, but then suddenly stops, and doesn't Zone Attack on C2, because the caster has no LOS to it, even though the Tsunami source point does.
I get the rules implication why, but throw this in my 'weird & unintuitive' bucket also.
If the caster dies, does the Tsunami continue?
If a Mage dies, continue play until the end of that Phase (e.g. the Upkeep Phase, or the current creature's Action Phase.)
The game does not end immediately upon Mage death, so Tsunami will not be stopped by that occurrence. However, regardless of whether the caster is a Mage or otherwise, death of the caster will fall into the realm of the caster moving, which we've already gone over. To reiterate though, this will also not have any affect on a zone attack in progress, but obviously the caster will not have the ability to target additional zones to attack after the current one is finished, so those will fail.
So even in the simple Scenario 1 above, the caster death in one zone, means that you finish attacks in that zone, but don't continue onto any other zones in the row. Again, consistent and clear from a rules point of view, but not at all intuitive - another example into that bucket...
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Now, consider an alternative: If you errata Tsunami something to the effect of "Tsunami is the source of these attacks" (possibly needs "attacks and pushes?)then
all of these oddities go away, and having cast the Tsunami, it goes off and does it's thing until it's natural conclusion, and acts in the way that players would expect (and for that matter that physics would imply).
What's the downside? The only change that you might get from this that I can think of would be to things like Reverse Attack and Redirect, but Tsunami being Unavoidable means they didn't have an interaction here anyway. Is there something that this breaks that I've not thought of?
I really think that Tsunami should receive errata to say that Tsunami is the source of all effects.