User blog comment:IamJakuhoRaikoben/Chapter 433 & 434: Ikusatsunagi & Demolition Fist, Review./@comment-63.152.122.15-20150511182921/@comment-24546844-20150511183503

I guess he meant that the souls are to summon Zeref. Ikusatsunagi was just a mean to kill them all faster.