The question that Iris had asked to Abby was; what happened if all the swords were broken? They didn't know what the answer, because there was no record for that. All the record about the history of Serafim and Decratic came from the first Serafim and it was Na, who created the five swords.
Therefore, Na didn't need the five swords to sustain her power.
But now, they would get the exact answer for that question and it was; the divine power that the Serafim had would disappear too.
What Abby had guessed was not wrong. A Serafim without her divine power would still be a Serafim, but what the used of her if it was her divine power that could destroy Decratic?
That was what Decratic aimed, that was what he wanted.
He wanted the Serafim for himself, but he feared her divine power that could kill him.
That was what this elaborated plot for.