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Reaction to mana deficiency highlights the difference in experiences between new and old hunters.
So mana deficiency is a very severe, established medical condition. The primary symptoms are mental instability, headaches, weakness, self harm, and potentially suicidal tendencies. I think Seo running out of mana and all his teammates thinking it’s so novel and cute at first really speaks to how new hunters truly have no idea what the Revengers and old hunters had to endure and how difficult it was.
The level of shock and horror when they realize he can’t be healed to restore his mana really highlights how different things are. The are in an S class dungeon but they weren’t mentally prepared for the risk of death.
Seo and Dorim exerted themselves to the point of mana deficiency almost constantly for most of the time they were in the great dungeon, and it was likely a common experience for the others, which likely explains a lot of the suicides in the dungeon.
Dealing with mana deficiency on top of trauma must also be incredibly severe. If it’s directly blamed for the deaths of two Revengers, they either must have done it very soon after getting out of the Great dungeon or they must have gone into other dungeons after. We don’t know when mana potions became commonplace.
It also really speaks to Seo’s strength of mind that he was able to endure it for so long, and remain functional afterwards.