It was a mystery how Kou possessed his targets, even now, but she had been told the basics of it through drawings. ‘If it’s that then’, Elmel joined the plan in order to get Kou close to its head.
Originally, she had planned to slow it down in the mountain, and then have everyone retreat to a safe location. Afterwards they would inform the adventurer’s society and have them send out their elites in order to kill it, but if Kou can posses it like the demon hound, then it’ll be safer that way.