User blog comment:SoulRise99/Larcade is the REAL Fairy Heart!?/@comment-31463848-20170309215005

I don't think that mavis and zeref actually had sex because in chapter 450 when they kiss, it shows that straight after that she dies, but there isn't really an explanation of why she had life inside her although because Larcade just died in chapter 525, it may give one but it could be possible that zeref created another demon inside Mavis to try to keep her alive (making her the demon) but because she was put in the crystal by Prect, a) how could she give birth if she was dead? b) wouldn't he be aged 400 ish years if when he said "let live" and somehow birthed larcade back then.

My thought is that fairy heart is Mavis because she was getting it extracted out of her by Irene, and Larcade is the child of zeref (much like how mard geer is/was)but being a demon that formed by zeref using mavis' power in a attempt to resurrect her. Although this is wishful thinking and I hope that this question isn't left unanswered as Larcade being the legit son of Mavis and Zeref despite being dead and frozen in animation in a crystal. But he is defiently not fairy heart