Well guess all those Mavis x Zeref shippers can be happy now. Unreal most assume Zeref probably created him. But I find it impossible for Mavis and Zeref to have a kid together. While I understand both are cursed and things that they love will die and such, so having sex without love is possible, and no love carrying a child is also possible. Most people will say "but Mavis is like biologically 12 she can't have a kid!", but concur she actually can as girls do hit puberty at a younger age.
But I still think Rahkeid conception is definitely by the use of magic. Mavis illusions and Zeref has an ability to make the illusion come to life. The whole Pleasure thing that happened last chapter can be sooo much different than just sex like most people were saying (masturbation, money, just sins in general).
It seems the author is now just throwing more and more curves out there. I'm guessing this arc is the end all of Fairy Tail then. But I will ignore this kid thing, and worry more about Eileen's relation to Erza.