Valeu and Calina didn't notice, but the Blue Pendant inside Calina's clothes was pulsing with a faint blue light at the moment. Back then, Calina's Blue Pendant's Sealed Array recognized that she was in danger, so it immediately activated. This happened while Calina, Erna, and Silo had just been surrounded by the Decaying Devourers.
Reans group had taken a few minutes before they started moving. However, Calina and her protectors also took quite some time to break through the Decaying Devourers' encirclement. They also used another few minutes before Valeu cut Erna and Silo's head. Last but not least, Calina's own struggle and the Surge's sudden start had bought her just enough time for Rean's group to arrive at the scene.
Now, Calina was saved in the very last moment by Roan's first not weapon-related skill, Shadow Bind.