Since each Lord Ravager of Destruction is serving a specific paths death, and Phainon is most likely supposed to be the one to end Nanook THEMSELF and the Destruction path because he is not necessary Iron Tomb but is born out of him, because Iron Tomb is Eruditions destruction
This means Phainon will join us on the Astral Express alongside Sunday who already joined permanently and Black Swan will depart as was leaked after Amphoreus
So get ready for Phainon and Sunday to stay for a while and have interactions together