With the death of Rei, the remaining Umbra Assassins fell apart.
Credit to the disciples of Rei, they didn't immediately collapse. By the time Anastasia and I had dealt with Rei, there were still ten left standing. My friends, on the other hand, were on the verge of collapse. They were running low on mana, and were basically on their last legs. The numerical superiority of the Assassins was winning out in the end.
I didn't blame them. I had spent quite a bit of mana myself, summoning my big guns such as Cetus and Hydra. Which, by the way, were still around. With the shadowy gargantuan multi-headed serpent gone, after its spellcaster was slain, Hydra could slither toward my friends to back them up. Cetus, though wounded, was still able to soar onward and envelop the entire place in a freezing field. Two Assassins were frozen solid before they even realized what was happening. The rest scattered, only to be seized by Hydra.