Reign's transformation was a last resort, an act of desperation in most cases. As the battle raged on, with the monster adapting to their every move, he knew that they needed to find a way to end it quickly.
They were managing to deal damage to it, especially with their marks activated, but even then, the damage they were dishing out to the monster was still not enough.
The monster was simply too difficult to kill, it was capable of healing its wounds almost as quickly as they were able to inflict them, and it seemed that no matter what they did to the monster, it was not using too much mana or any other weird kind of energy that the corruption used.
They had been fighting for quite a while, and there were no signs that the monster was going to slow down, none at all.
It was going after them without caring for itself at all, without caring about what would happen to it, because it didn't need to care, it would just heal up right away.
