In the solumate au ace not realising hes latched on to nyx because it happened when they were so young so angst because while nyx doesn't know the soulmate thing but ace does so ace is upset because he loves nyx but he doesn't know nyx is his soulmate and that kills him that as far as he's concerned nyx isn't his soulmate. Jump to Regis meeting nyx after reveal and being all "so nice that you already found your soulmate" cause he can feel it and ace is just like ???!!!??? Nyx=Soulmate ???!!!???
...Bonus the Reveal happens because Noctis meets Nyx face to face and can feel the magic of another Lucis Caelum swirling around Nyx and buried within him so deeply that Regis’ magic feels like a guest. Like Noctis’ father is being allowed to grant Nyx magic.
So Noctis, kinda freaks out a bit, because as far as Noctis knows there are no other Lucis Caelum’s, and he has to decide what to do and Nyx is on ‘Guard the Prince’ duty, so Noctis wanders into Regis’ office and when Nyx goes to wait outside Noctis grabs his wrist and drags him into the office.
“Dad,” Noctis calls as soon as the door is open and he drags Nyx in, “Explain.”
Regis looks at his son, then his attention is turned to a confused Nyx and like Noctis, Regis can feel Ace’s magic in Nyx.
“Oh,” Regis goes, “This is interesting.”
“Interesting,” Noctis says, “Is that all you have to say? There’s another one of us out there! And we don’t know where they are!”
“I’m sure the Glaive would be happy to assist us with finding out who this missing Lucis Caelum is,” Regis says, “What is your name, Glaive?”
“Nyx Ulric, Your Majesty,” Nyx says, keeping his face blank, “May I ask something, sir?”
“What do you mean by missing Lucis Caelum?” Nyx is then treated to a crash course on magic and soulmates and how someone’s magic has already claimed Nyx and that Regis’ own magic is being treated like a guest.
When Nyx is asked if he has any idea who this missing Lucis Caelum could be he thinks of all the seemingly impossible things Ace has done and drags a hand down his face.
“I think,” Nyx says, “That I need to call my husband.”