Loss of mana causes mental exhaustion and eventually pain because the “Organs” that hod mana are a part of the soul not the body and when is gets low it is like having an empty stomach and the feeling of “hunger” is mental exhaustion because it is found within the soul not the body and that is the closest interpretation your soul could find because it wasn’t a natural part of the soul and thus couldn't create a separate feeling for it. The later pain comes because this organ needs mana to maintain itself thus if you run out of mana this part of the soul can collapse. The consequences can range from just a temporary recovery period with incredible pain, if you're lucky, to loss of ability to use mana, to a complete soul collapse. As you get stronger the risk of the last one grows greater because at first these organs are just kinda attached to the side of your soul but as you grow stronger and your soul grows larger it gets incorporated further into your soul and becomes more intertwined with the rest of it making the collapse worse.
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