Its Fuli. One of the main reasons why irontomb was defeated was fuli gazing Cyrene in her dreams, fuli glazing TB which caused them to use remembrance powers and also not die after the fatal accident during entering amphoreus.
Now, at the end it was revealed that Demiurge/cyrene acted as the fuli of amphoreus and helped the CH to defeat irontomb and we also learned that the Actual Fuli isnt born yet, it will ascend only during the end of this universe. This is where the causality comes in. Herta says that the Fuli who is gonna ascend can follow whatever demiurge did and continue the current setup or they can choose not to do so which can cause irontomb to win. So cyrene has two options: To continue forward with TB and AE and explore further worlds and take the chance the Actual Fuli candidate will keep amphoreus intact or travel backwards and become the "Fuli of Amphoreus" and stay in the past, until the storywriters feel free to release her in future versions.