How do calamities work, again?

[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.

[Ilenya]

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

So not only were Ardbert and co too efficient, they were also too heroic.

That's hilarious

[JeanneOrnitier]

Is that from the fifth role quest after doing the first four?

[Berethos]

Is that from the fifth role quest after doing the first four?

That is where it comes from.

I've not quite yet reached the point to get my own screenshots of it all (leveling Summoner to get both caster and magic options opened, then DRK to get the tank one done, so it shouldn't be too long before I can add it to my growing screenshot collection), but I didn't mind reading up on information provided by someone who had made it through the entire role quest series, and that was among the information they shared.

[Zohar_Lahar]

I don't have quotes at hand, but from what I understand the Seventh Umbral Calamity was indeed a Calamity with the associated Rejoining.

I think the Rejoining occurred when Bahamut started breaking free of Dalamud. For millenia it had been fettered by the neurolinks, clearly designed to withstand the Dreadwyrm's power. Once the Seventh world started rejoining, that cascade of aether probably enhanced his megaflare to the point of overwhelming the satellite, causing Dalamud to violently disintegrate. And since the Seventh shard was overaspected to Darkness -- all six elements astrally charged -- this also augmented the secondary effects of the impacting debris and rampaging primale to causing powerful megatsunamis, rifts opening up in the earth, forming glaciers, and other crazy effects.