Well, it does seem like some people are confused from the fact that Zeref is emperor Spriggan. Honestly, I did feel it came out of nowhere but i gave it a thinking and for me, now it makes perfect sense. This is what I came up with:
- The question that was raised a couple of chapters ago when Arvalez was introduced was why didn't they attack when FT was defunct or 8 years ago when they were weak? The answer's Natsu. Since Zeref was the emperor, he was certainly worried that carelessly raiding FT would hurt Natsu. And when they were kinda killed 8 years ago, it would feel unfair. Somehow Zeref must've known Natsu would live when Fairy Sphere was activated and stealing FH would not contribute to his "despair" since Makarov would shut any attempt of Natsu going for revenge to retrive it. He also might've not known about the disbanding of FT until lately, since only after learning that he wants to go for FH
-And why doesn't anyone know he is emperor Spriggan? It might be because he's like Gildarts, i.e. a nomad, for once he's here for a moment and then he's gone. And yes, it is also because he has an alias "Spriggan" that only works in Arkatacia and in Ishgar, it's only for name sake.
(I'm adding that it might've been Zeref himself who activated Sphere in tenroujima. Since he finally saw Natsu there, he might've been formulating a plan to end everything, ACNOLOGIA, HUMANS and HIMSELF by putting it all together and hence planning a 'FATE' for Natsu. This is also because he has shown interest in FH not so long ago.)