The demons up top weren't expecting me to come from below. I had the element of surprise. I ascended the stairs as I quietly drew the Unmaker. I took to the shadows when an Imp patrolled by. It lingered for a moment, but it carried on, undisturbed.
I finished ascending the Hellish stairs. Up top, I selected a target for the Unmaker. Once I found the unlucky demon, I fired. I had previously found the third and final demon key and had inserted it into the Unmaker. I didn't know what the weapon could do when fully-upgraded until now.
The Unmaker's beam found its target and melted it. However, the beam wasn't changed. Instead, the weapon also emitted two demonic souls that sought out their brethren of flesh and blood. I watched as what had been a cohesive horde dissipated into a clamor of panicked Hellspawn. In a matter of mere seconds, the horde had been slaughtered and afterwards, the souls disappeared, satisfied with their meal.
I now understood to the fullest extent why the weapon had been hidden away. This weapon, when fully-upgraded, was truly formidable, even rivaling the BFG. The best part about the Unmaker was that the ammo was plentiful: the harvested souls of demons. The Unmaker emitted the wails of the many, many demon souls I had harvested.
I scoured the castle's ramparts for any stragglers, and found none. I jumped down from the wall and into the courtyard of Castrum Deimos.