As soon as they did, the Hell's Gate automatically shrank and transformed into a black bullet.
To be more specific on the nature of this bullet, the Hell's Bullet was used for detaining and torturing an adversary.
A prisoner would spend an entire day trapped inside the box with the monsters whose numbers corresponded to the number of crimes committed by the detainee before their ensnarement.
Once the day was up, the Hell's Gate would have finished serving its purpose which is to torture and steal from its prisoner. The person imprisoned would lose all his mana, a part of it would be transferred to Alex while the other part would serve as fuel for the gate to work. A nasty bullet.
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