User blog comment:Water Dragon Slayer/DREAM BATTLE CHRONICLES: SECOND MATCH!/@comment-11966702-20130726045815/@comment-4829046-20130726050913

Nothing. They cancel each other out XD Both are meant as Magic that brings death and I suppose the two of them are resistant to it or at least Zeref is. But in technical terms, the Death Pillar can do more actual environmental damage cause even though Fairy Law covers a large distance it only affects people. So if Zeref is unaffected by Fairy Law and Mavis is unaffected by Death Pillar the environmental damage will still be a handicap for Mavis.