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.