By the time Arsinoe returned with Typhon, it was already too late.
When they came into the underground, they saw Morgana laughing. This was especially hard for Arsinoe because it took all of her strength to bring Typhon back and now her injuries were more severe.
But she should've remained here because it seemed her efforts were all for nothing. If she had stayed, she could have interrupted the process, because according to Typhon, Morgana had done something to the hatchlings which meant they would only follow her in the future.
Morgana saw her dumbfounded expression and turned to look at Typhon.
[What can you do now?] She laughed mockingly. [You should return to your plans, because you're too late. They belong to me now.]
Typhon took a look at the spacious underground and the collapsed stone barricades. Without speaking or acknowledging any of Morgana's words, he immediately started to attack.