Isterio said:
Guys I can´t believe that seriously no one thinks about it?
Natsu was brought back to "live" by Zeref. However it seems to be an incomplete form of revival since his live is tied to E.N.D
So I´m assuming now since Mavis talks about a sin, she did the same thing, but suceeded. Rememember when Makarov said how Mavis beat Zeref in the past?
The conclusion is probably that she brought back her best friend back to live.
Something Zeref couldn´t succeed in, but she used a different concept, one that requires love.
That´s why her sin is to be crystalized, it´s nothing powerful just something that Zeref cannot afford to lose.
God Serena is Yuriy who also attempted an imperfect magic and is in the same condition as Mavis, which is why he still looks so young. After his perfomance in the year XX70 he started calling himself Serena.
The reason he sides with Spriggan/Zeref is because he promised him to bring his beloved Mavis back, who may or may not be Makarov´s mom or his best former friend.
God Serena is not Yuri. He looks too bloody young while Hades and the tree dude are all old men.