The arrows made of death energy pierced through the barrier but only the arrowheads done so. The arrows themselves did not reach the God Izanagi, but it did put him in high alert. Arrows that could pierce his Magatama Barrier that is made of life energy could only mean that it is of the opposing element. He already knew of dark elves before as they are often called as the Cursed Forest Children, but it seems that the rumors were true.
"A being that could wield death energy while still having vitality in them." The God Izanagi murmured as beings that could wield death energy without any repercussions are rare.
