Here, there is no war-there are no sides. Once at the doorway to the inner area, she was approached by another disguised cultist. Kassandra pushed open the doors, leaving Herodotos behind to descend into the lair. Kassandra changed into the cultist's cloak and mask she had retrieved from Elpenor's hideout. I don't think they'll look too kindly on that. Herodotos awaited her outside the chamber, and together they walked to the entrance.
With Elpenor successfully assassinated, Kassandra made her way back to Herodotos.
Kassandra sought out the lair of the Cult of Kosmos and infiltrated their meeting.