There was this bond between the Serafim and Decratic. As long as one lived, the other would exist too. If the circle of the Serafim continued, it meant Decratic still existed as well.
And Iris determined that she would be the last Serafim, which meant, she was going to destroy Rei. She was not sure how Rei could tangle himself with Decratic, because from what she remembered from Na's memory, Rei was loyal to her, there was no way he crossed the bridge and asked for Decratic's help.
There was only one reason if Na didn't have any recollection of that, it meant, Rei crossed to the other side after her death.
As if to answer that, the moment Rei tainted Iris with his dark magic and Iris absorbed it, all the memories of Rei's life burst in her mind. She could see what happened. She saw what Rei had done to her mother and many more…
