What I Saw He Had Done For Me (Nalu fanfic)

Natsu has vanished, leaving Lucy and Happy to wonder where he went. after he left 3 years ago, on Lucy's 21st birthday, he suddenly comes back. Why has he started acting weird towards? Why does he seem like darker person? What does it have to do with Lucy?! **MARTURE, SEXUAL, AND AGRESSIVE(seeming abbussive) CONTENT! READ AT YOUR OWN RISK!**