How do calamities work, again?

[Daralii]

More to the point, Floods weren't caused on any shards except the First and Thirteenth - Floods are counterproductive to the Ascians' goals.

The trick is to tip the elemental balance toward an element without causing a Flood, so that the excess elemental aether from the shard will trickle into the Source. After this has gone on long enough, it cracks the barriers between the shard and the Source, after which a Calamity can be used to collapse the shard and return all its aether to the Source.

Floods occur when the barrier isn't cracked, but the elemental aether has built up to catastrophic levels on a shard.

We know that the barrier was cracked on the Source because it was stated that the thinning of aether, which was previously stated to always preclude a Calamity, was a result of the Light seeping in.

[Ilenya]

More to the point, Floods weren't caused on any shards except the First and Thirteenth - Floods are counterproductive to the Ascians' goals.

People keep saying the Flood here was counterproductive to what the Ascians want, but I don't recall that ever being said. Does someone have a source for this?

[Jandor]

People keep saying the Flood here was counterproductive to what the Ascians want, but I don't recall that ever being said. Does someone have a source for this?

Yeah I didn't get the impression the flood on the first was counterproductive, indeed if I recall correctly Emet initially was all "you've messed up my plan" before deciding to chill out and watch.

I don't think it's the flood itself that's the problem, you just have to make sure things are in place to get the flood to where you want it to go, and Ascian plans on the source to get it there were working out fine.

[JeanneOrnitier]

People keep saying the Flood here was counterproductive to what the Ascians want, but I don't recall that ever being said. Does someone have a source for this?

It would have been if Minfilia hadn't stopped it back in Heavensward. Their plan in ShB was to use the Flood, but that's only because Minfilia stopped it. If she hadn't done that it would have consumed the entire First and rendered it just as much of a useless void as the Thirteenth. Remember, Elidibus told Urianger about the Flood of Light which is why Urianger got involved in Ardbert's crew to stop it. The Ascians needed Minfilia to intervene back then as much as the people of the First did.

[Cilia]

We know that the barrier was cracked on the Source because it was stated that the thinning of aether, which was previously stated to always preclude a Calamity, was a result of the Light seeping in.

Yes but this only occurred during the tail end of Stormblood (Shadowbringers' prologue, essentially); the Flood occurred a century ago on the First, before the Warriors of Darkness sojourned to the Source in an effort to stop it or cause a Rejoining so their existence wouldn't be wiped out entirely. Minfilia's intervention halted the Flood, but the elemental imbalance on the First was still tipped dangerously in favor of Light over Dark, which would have led to the Eighth Umbral Calamity (Rejoining) if not for Crystal Exarch G'raha's intervention.

The barrier isn't cracked during Heavensward, which is when the Flood was an immediate threat to the First (as opposed to the Sin Eater infestation we deal with in Shadowbringers).

People keep saying the Flood here was counterproductive to what the Ascians want, but I don't recall that ever being said. Does someone have a source for this?

Elidibus mentions it at some point during 2.1 - 2.4, during the Warriors of Darkness' mini-arc in Heavensward. I don't have the exact line of dialogue, if that's what you want, but it's there.

[Ilenya]

Elidibus mentions it at some point during 2.1 - 2.4, during the Warriors of Darkness' mini-arc in Heavensward. I don't have the exact line of dialogue, if that's what you want, but it's there.

Mostly what I am seeing is less "Flood is bad" and more "Flood is poorly timed" which for the Ascians amounts to the same thing, but it's useful to at least have some sort of clarification on it. I imagine as a whole the Floods are what they want, but it needs to be set up properly ahead of time. Make the cracks, then let the Flood happen. Ardbert and co just did their jobs too well.

Was it mentioned how exactly these cracks were made? Were they just events leading up to it that allowed it? So for Bahamut's, things like what became the Burn would be starting signs of the Seventh crack, while for the attempted eighth it'd be...Shinryu/Omega due to timing, I guess? Or is this all stuff that happens behind the scenes and not noticeable until the aether starts to change.

Edit: I realize that the cracking could have been the Battle at Silvertear Falls for the Seventh Calamity. Probably fits better in the timeline, but still hard to say.

[Daralii]

Mostly what I am seeing is less "Flood is bad" and more "Flood is poorly timed" which for the Ascians amounts to the same thing, but it's useful to at least have some sort of clarification on it. I imagine as a whole the Floods are what they want, but it needs to be set up properly ahead of time. Make the cracks, then let the Flood happen. Ardbert and co just did their jobs too well.

Was it mentioned how exactly these cracks were made? Were they just events leading up to it that allowed it? So for Bahamut's, things like what became the Burn would be starting signs of the Seventh crack, while for the attempted eighth it'd be...Shinryu/Omega due to timing, I guess? Or is this all stuff that happens behind the scenes and not noticeable until the aether starts to change.

Edit: I realize that the cracking could have been the Battle at Silvertear Falls for the Seventh Calamity. Probably fits better in the timeline, but still hard to say.

Likely the battles for Ala Mhigo and Doma, as well as the rebellions that followed in their wake. I think the thinning of aether was only brought up after 4.0.

Going back to Heavensward, Ardbert was pissed at Elidibus because the First didn't have time for the gradual ramp up toward a calamitous summoning, so Urianger proposed that there was no surer way to instantly swing the scale far enough than to kill the realm's beacon of hope. It was obviously to manipulate Ardbert's group to bring six crystal bearers together and stop the Flood, but considering Elidibus had arrived at the same conclusion by 4.56(after Gaius and his group coincidentally destroyed production facilities to push back the use of Black Rose), I doubt that was a flat out lie on Urianger's part.

[Cilia]

Was it mentioned how exactly these cracks were made? Were they just events leading up to it that allowed it? So for Bahamut's, things like what became the Burn would be starting signs of the Seventh crack, while for the attempted eighth it'd be...Shinryu/Omega due to timing, I guess? Or is this all stuff that happens behind the scenes and not noticeable until the aether starts to change.

Edit: I realize that the cracking could have been the Battle at Silvertear Falls for the Seventh Calamity. Probably fits better in the timeline, but still hard to say.

Crystal Exarch G'raha explains the process of Rejoining in detail after you collect Alphinaud and Alisae for the Shadowbringers campaign, but to put it shortly, there is no singular event on the Source or Shard that signifies the barriers as having cracked. Finding this out is simply a matter of observing aetherial activity, as the excess elemental aether creates disturbances on the shard and Source both. The excess Light on the First led to the Flood of Light (a century ago), the infestation of Sin Eaters, and the perpetual daytime that lasts until the Lightwardens are vanquished; on the Source it was observed by the thinning of aether, evidenced by deaspected crystals popping up (as excess Light aether essentially nullifies aetherial activity). The effects of that elemental aether are also magnified on the Source while the shard is imbalanced, which is why the Black Rose poison was so potent in the bad future Crystal Exarch G'raha hails from.

"Cracking" the barrier between Source and shard is simply a matter of letting the excess elemental aether from a shard trickle into the Source for long enough. This eventually causes a Calamity on the Source, which is a result of the shard collapsing and all the excess elemental aether being unleashed upon the Source.

To begin at the beginning, then... In the ancient past, a single star was divided into fourteen worlds. [...] Though physically separate, they retain a connection to each other, and the Source especially. Now, let us assume that a given element in one of the shards attains abnormal ascendancy. Just as water will flow from the highest point to the lowest, the excess energy will begin trickling into the Source... And such an influx of aether will of course exert a palpable influence. If the element in question were fire, then drought and wild fires might ensue. If it were ice, one might expect the weather to turn bitterly cold. As aether continues to pour in, such phenomena will become more and more extreme, until eventually, a single untimely event triggers a disaster which "cracks" the barrier dividing the two worlds. What was once a trickle now becomes a deluge, sweeping the shard along to be rejoined with the Source. At the same time, the element which held sway in the shard is unleashed in full, its energies amplifying the original disaster to truly catastrophic proportions. An earthquake thus magnified might strike with enough force to shatter continents; a tidal wave might swell to a size capable of drowning entire nations. These devastating events are what we refer to as "Umbral Calamities."

This is the clearest picture we have of the process of Rejoining.

[CaesarCV]

Big spoilers for Shadowbringers below!

To be fair the Ascians' plan wasn't to let the flood overtake the First. That was actually something of an accident. Their plan was probably more conventional. Tip the elemental Balance of the first to Light, and then cause a calamity so it could get rejoined. However, when Ardbert and pals killed the Ascian in charge of said plan, something went wrong and the light started rising to catastrophic levels, at risk of causing a flood. That was why Elidibus got their assistance, so that they could basically cause an emergency rejoining before the flood destroyed the whole star. They failed to cause a rejoining, but Norvrandt was saved due to the actions of Urianger and Hydelyn. If the flood had happened without being stopped, the First would likely have become useless to their goals.

But the flood was stopped, leaving the Ascians with a world to rejoin. Emet-Selch's plan was then to use Vauthry to create the conditions for the Light to leak into the source, and then use Black Rose as a catalyst for the calamity allowing for the First to be Rejoined with the Source. When the Warrior of Light and pals started to disrupt things he altered his plans though, first trying to ally with them but keeping other options on hand as a backup. If the Warrior of Light couldn't contain the light, he could just go forward with the WoL taking Vaughry's place and the First able to be rejoined. If the WoL could, he would get them/The Scions on his side and then figure out the best way to rejoin the shard.

[Berethos]

Big spoilers for Shadowbringers below!

To be fair the Ascians' plan wasn't to let the flood overtake the First. That was actually something of an accident. Their plan was probably more conventional. Tip the elemental Balance of the first to Light, and then cause a calamity so it could get rejoined. However, when Ardbert and pals killed the Ascian in charge of said plan, something went wrong and the light started rising to catastrophic levels, at risk of causing a flood. That was why Elidibus got their assistance, so that they could basically cause an emergency rejoining before the flood destroyed the whole star. They failed to cause a rejoining, but Norvrandt was saved due to the actions of Urianger and Hydelyn. If the flood had happened without being stopped, the First would likely have become useless to their goals.

But the flood was stopped, leaving the Ascians with a world to rejoin. Emet-Selch's plan was then to use Vauthry to create the conditions for the Light to leak into the source, and then use Black Rose as a catalyst for the calamity allowing for the First to be Rejoined with the Source. When the Warrior of Light and pals started to disrupt things he altered his plans though, first trying to ally with them but keeping other options on hand as a backup. If the Warrior of Light couldn't contain the light, he could just go forward with the WoL taking Vaughry's place and the First able to be rejoined. If the WoL could, he would get them/The Scions on his side and then figure out the best way to rejoin the shard.

To add to this (some major spoilers from some Role Quest revelations):

The original plan was that a specific individual involved with Ardbert and his crew would lead them on into situations where they would generate Crystals of Light through selfless acts of various types, but they would also be contributing to these ever escalating situations. They would then reveal themselves to the WoLs of the First after all but one had gained their crystal, with the intent being that in anger at that betrayal Ardbert would strike them down and not gain his own Crystal of Light...leaving the world at the necessary brink but not tipped into Flood territory.

They underestimated Ardbert though, as he chose not to execute them for their betrayal and instead went after the ones that had manipulated this individual...the actual Ascians. Upon defeating them, Mitron and Loghrif, he gained his own Crystal of Light...and that act tipped the balance too far and lead to the Flood of Light that Minfilia would later stop.

That's the quick version of the details regarding their actual plan - I've left some of the other major revelations from that unspoiled, as they don't necessarily directly relate with the causing of the shift toward Light and the Ascians basic plan to bring the First to the brink.