So as it turned out, the lyrics to Kainé were a made up amalgam of languages ranging from Gaelic to French to German with a heavy Japanese filter applied for roughly half the pronunciation. In order to know what they mean, aside from Kainé itself showing up once in the second line of the second stanza, we'd have to ask the linguist singer they got who created it.
Of course, this had me tumbling backwards to my adolescence. For me, I started this series with Drakengard(Drag-on Dragoon in JP) back in 2003 at the age of 13. I took turns with my older brother, who was 22 at the time. The battles were massive, and the conflict was a small union of kingdoms versus an evil empire, as is the standard trope in JRPGs, action or otherwise. In this game there were 5 endings, all extremely brutal and twisted, with even the good endings featuring death or pain for the heroes that you play. However, the only ending I ever got to see was Ending E, the final ending as I know it. My brother would stay up late into the night, beating the game over and over again, and he would describe the endings to me if he thought they were suitable for me to hear. 4/5 weren't, and I've watched them on Youtube to know why. Those babies sure can eat a lot.
In Ending E, Caim and his pact partner, Angel(U.S.) resolve to fight the Queen Beast, an all white giant humanoid woman, who has become pregnant by absorbing time as an energy source in order to destroy the world. Upon reaching her, it destabilizes this energy, and they along with the Queen Beast are thrown forward in time to modern day Tokyo. They fight, with the Queen Beast remaining immobile and no longer pregnant, whereupon she screeches the Power of Song out in alternating black and white spirals of that ancient text of God or whatever.
It's been so long I can't remember, but I guess dragons can answer with the Power of Song as well? Because that's what you do, white to cancel white, black to cancel black. The mechanic in the Her Inflorescence fight is a reference to this (funny, cause it didn't trigger any memories for my brother). Once you defeat her, she's expended all of her power and disintegrates into flaky white dust. Angel and Caim are then promptly shot down by missiles fired from Japanese Defense Force jets, F-15s on loan from the USA, iirc. Angel falls and is impaled on the Tokyo Tower, and you see this at the end of the credit scrawl.
After that, this ending causes the events of both Nier games to happen. I guess all three, since Nier leads to Nier: Automata. A disease caused by the entry of magic into the world again wipes humanity out, and we create a network to store our memories and Replicants to be bodies for those memories. The Replicants gain their own consciousnesses though, resulting in actual humans(Gestalts) still vulnerable to the Black Scrawl magical disease, and their replicant bodies. By 4198 all humans are deceased. About 1,000 years later in 5012, Aliens invade using an army of Machine Lifeforms to conquer the Earth.
Their conquest is rather easy, as the replicants/androids leftover shut down their factories at some point prior to the invasion, and also had a civil war of sorts with an entire faction no longer wanting to be bound to humanity settling in Australia. The machine lifeforms have a similar desire to this faction, though their animosity is directed at their Alien overlords.
In 11306, the Machines succeed in eliminating the Aliens while keeping their own interstellar Machine Network intact.
In doing this the entities known as Red Girls sought to have Machines transcend their programming directive, and they allowed Machines to start breaking away from the network, so that they could expand machine lifeform's evolution and knowledge base. If they had smashed the androids as they easily could have, controlling over 4/5ths of the planet, they would have shut down. The entire point of Nier Automata is to tell us the tale of these entities, as they manipulate events to boost their own consciousness. In the end, they do. They also alternate between speaking with female voice and a deep male one.
So I guess the FFXIV segment of Nier takes place after the E Ending, where the Pods have rebuilt 2B, 9S, and A2. 2B and 9S joined the Machine Network, and I guess they had a leg in, because somewhere along the line, the androids took up machine parts for their android's black boxes. The Machine Network is interstellar, and comes to The First, where it produces machines to explore and possibly to conquer, but again, somewhere along the line the Machine Network got ahold of the Watcher's magical powers from Drakengard, and have incorporated it into a few of the bosses? I'm not the biggest Nier fan, so I'm not sure about a lot of details.
It seems like the machines ascended to such a high point that they were able to recreate the Seeds of Destruction/Resurrection, but out of that attempt we get the False Idol, a physical manifestation of the Red Girls, which then proceeds to be taken over by The Flower from Drakengard 3, and is mixed in with 2B(real designation 2E) who at one point was mixed in with A2 who in the YorHa Stageplay was said to have been given the memories of Kainé. The Flower from Drakengard 3 is what allowed the events of that game to take place, and it's a prequel to Drakengard, somehow. It was capable of reviving beings and making them into Intoners, beings capable of wielding the Power of Song. Which is like an ultimate godly power, that can kill, empower, and do many other things just by singing it.
So I guess in the other world, the Machine Network became all powerful, and the final boss at the tower was its extension into The First. 2B and 9S have made it their mission to try and contain it to that universe, I guess? Or I suppose once the Machine Network triumphed over man's last remnant, mayhaps the God and its Watchers(Angels in JP) took it over to use an instrument of destruction across all universes?
I doubt we get a concrete answer. It seems like every time I unearth another part of the story, it just dredges up more questions than answers, since Yoko Taro largely creates emotion driven artwork rather than completely thought out stories. Ah well, it was fun to ruminate on for some hours of my life.
Oh yeah, as a fun aside, the Flower and beings renimated by it can only be killed by dragons or weapons made with dragon parts. So doing the raid on Dragoon is kind of poetic.