Mana definitely was a wonderful thing that the humans got blessed with, but each thing has it's own ups and downs.
The "ups" of mana allowed humans to produce cleaner fuel, develop faster, evolve their bodies etc.
And as for the "downs" , well the biggest "down" definitely was the arrival of the demons.
The demons - one of the most cunning and blood thirsty species of the universe - had come to earth roughly 500 years after the arrival of mana.
Their aim ? to colonise the earth, use it's resources for increasing their population and strength and make the humans their food.
And as to how did the demons come to earth ? Through spaceships ? NO
then through an intergalactic wormhole ? Definitely No
But rather the simple structures known as DUNGEONS.
Yes, the same dungeons that people had taken control of, who the humans use to train themselves and earn money.
Now one needs to think about how the dungeons were like TARDIS (doctor WHO reference) - ie , smaller on the outside but bigger on the inside ? Simply because the dungeons existed as a reality in some other world and these dungeons acted like a door to that world.
But then here comes the question ? if dungeons are like doors to other worlds then when humans enter them why are humans limited to a fixed space ? (for example - Will was confined to a room in the goblin dungeon, while the whole class was confined to a big forest which also had its physical boundaries) - It is because of mana capacity.
Mana was simply an amazing thing - each dungeon had a limit to it's mana capacity - the higher the mana - the bigger the physical space in which humans could travel.
Further it was this mana capacity only that decided the dungeon Rank and the rank of the monsters that could be inside it. So a Rank A dungeon implies it has a lot of mana as compared to a rank B dungeon, similarly Rank B> C>D>E>F>G.
And only because of this mana capacity only the monsters inside the dungeon were limited to be of that rank. So a Rank G dungeon only had Rank G monsters and so on.
Also this mana also worked in such a way that their was a one way entry only - ie - humans could enter the other world of monsters through them but the monsters couldnt enter the human world through these.
Coming back to how demons could enter the human world through Dungeons ? Simply by a dungeon break. A dungeon break was a situation where the dungeon collapsed due to mana overload. And when such a thing happened the dungeon reverted from a one way door to a 2 way door - ie - it allowed the creatures on the other side to come to human world.
Thus if there is a demon closeby on the other side when dungeon break happens then it can also enter our world along with the monsters.
Further demons had another thing that made them powerful - the power of soulbind.
Soulbind could be explained as some type of symbiotic relationship that demons could have with another being. To put it simply if a demon did soulbind with another being then both their souls got connected. The being just by doing a soulbind could increase his growth potential and become stronger faster.
Also as the being grew stronger, so did the demon. Thus the soulbind was like a positive for both the demon and the soulbound being.
But at the same time it also had it's downsides. For one, a demon could only use soulbind with 1 being, thus as long as that being was living, the demon could not perform soulbind with another being.
Moreover, if the soulbound being died, then the demon also received a huge backlash leading to lots of injuries.
But with this power, they had infiltrated the humans and had made many humans turn to their side too.
And one such human was standing in front of Will right now. Devin.
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Devin had a photo in his hand.
It had a beautiful girl in it. She had red hair and one of the brightest smile.
"Emily....." Devin smiled.
Devin, who while growing up was hailed as a genius by everyone, definitely had developed an arrogance.
This arrogance had increased further when he joined Divine, and why shouldnt it ? Divine afterall was the best academy of humanity.
As such he dreamt of becoming the strongest student, then the strongest hero of humanity who would kill the strongest of the demons and monsters.
But all his dreams and his arrogance shattered inside the academy.
Divine which was the best of the best, also had the best of the best students - the most strong, the most intelligent, the people with strong backings and thus here Devin became one of the bottom rankers of his year.
Yes outside the academy he may be a bigwig, but inside ? just an average bum.