The mana stone attack came as a surprise even for Park Shin Hye. It was common knowledge that beings from upper floors doesn't get affected by mana stone bombs. The way a mana stone bomb works are that when it detonates it releases the pressurized mana stored inside the stone. It may be able to harm people from 1s floor. But people on the upper floors have such strong physiques that detonation from mana stone bombs does not harm their bodies.
But it didn't mean that mana stone couldn't be used as an explosive. If a person can change the neutral mana inside the stone to a particular affinity then those types of bombs are disastrous to every type of being. But such bombs are very hard to come by.
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