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

Arlock said that souls were needed to summon Ikusatsunagi. Since not all of Avatar seems to have been sacrificed it could be that Ikusatsunagi wasn't at full strength.