Revelations from Patch 7.3: The Promise of Tomorrow

[Cleretic]

Calyx knows things.

So we know Calyx summoned Necron using a capital C concept, so we could assume he had control over the finer details like what Necron looked like or even what spells or attacks it uses. Necron used a spell "Grand Cross." Only two other beings have used Grand Cross in FFXIV: savage Neo Exdeath, and Ultima, The High Seraph or at least a version of it. One could just write it off as 'that's what it used in FFIX' or 'it's just a reoccurring big bad spell,' but we have to consider that this Necron was made from a blue print by a character, and so its ability to use Grand Cross means something. Calyx knows about the spell Grand Cross.

I don't think this is actually a clue, looking at the actual fights.

First of all, all three of those bosses use Grand Cross in their origin game, and crucially is basically the same in all three: laying down a truckload of status effects. They're actually three of only four bosses that use the ability (the fourth is the final boss of Tactics A2), and in each example is one of the defining attacks of the fight; it'd feel incomplete to not adapt Grand Cross in every case.

In XIV, all three bosses use Grand Cross completely differently; NED retains the status effect pile, but Ultima's Grand Cross drops the crystals that slowly explode into Bomberman-style plus signs, while Necron's is the stage-morph that creates rotating orbs that shoot lasers. The only connective tissue between the actual attacks is that Ultima and Necron's versions of it make cross shapes, but even then, they do it differently: Ultima's crystals emit the plus-shaped blasts, while Necron's orbs fire intersecting lines.

If we really want to ascribe worldbuilding meaning to every single version of this attack (which I should note is not something the game's ever done before this outside of Ultima the spell, which I think actually does do the same thing every time), I think the simple fact is that they're all named Grand Cross because they all have independent cause to be named Grand Cross.