1.23b/c Speculation

[Aistalia]

Well, during the cutscene you see after defeating Darnus you clearly see:

[Spoilers ahead, but I guess anybody's reading this thread won the fight already]

Before "dying" he extends his arm towards Dalamud.
His body dissolving into some steamy energy aura/aether, and it slowly flies to Dalamud. There's a scene just for that:


Now the moon/meteor/machina and the sinister thing that lives inside it has probably Darnus' will. They are finally one thing, he sacrificed his own "flesh" for Dalamud.
When you get back to Gridania and talk to Cid, he says he believes Dalamud is not going to fall and that Louisoux doesn't need to use the "last resort he kept muttering about...".
We clearly see he's going to use "it" in the trailer, sending the adventurers into a safe place at the end of time, and making them come back in 2.0 world, after a (long?) timeskip. Probably sacrificing himself. Something is going to happen in the next patches.
Although I believe the Darnus "hard" fight is going to be just to give us an extra challenge and probably unrelated to the storyline.