Chase's backstory. Due to being the test subject for The Empire, he has lost all memories of his past, was forced to become a glorified mindless killing machine until Scarlet's group eventually taken him in.
Ezekiel's childhood. Because of his demon blood, the majority of it involved him shifting from his usual kinder self into a violent beast that would cause such carnage that even his own mother was frightened of those outbursts.
The fate of the Republic of Grey: razed by Lucikiel, countless humans killed by the hordes of demons and the princess of the Republic Jane dying only to be resurrected as an undead by the demon army.
During the war a century prior to the events of the main story, Dark Dragons (demons created to slay the dragons) killed and devoured countless actual dragons. A small number of the surviving dragons have gone insane from the rage and shame, which resulted in them turning into Mad Dragons, effectively becoming nothing more than giant monsters driven by their destructive instincts.
The backstory of The Girl: watching the destruction of her home, and the murder of her family right in front of her, not to mention herself almost getting murdered as well.
Eventually proven to be the case: In spite of losing his wife and his daughter, Crow maintained a stoic facade, to the point that people around him were bewildered by his coldness and critical of his external apathy. In reality, he shed Manly Tears while no one was looking.
The whole situation with the Shadow Hilt sisters: Erze gradually becomes corrupted by her powers and has since become a murderous horror that lost almost all of her humanity by the time she meets her older sister Seria.
The origin of the Lu Estrona Dice currently used by Esker: First used by Hexipolia, the Wandering Illusion Clown. Hexipolia was a renowned mage and artist in ancient times who traveled around the continent to give boost the morale among the people and help them cope with the harsh reality. The performances were a major success, and the people whom he helped went as far as to make made the Moon of Hexipolia after he passed away in honor of his benevolent actions. Sadly, the side effects of his craft slowly started taking its toll, leading Hexipolia to suffer from loneliness and depression. While he initially considered ceasing his performances, he ultimately resolved to continue them. At that time he made the Lu Etrona Dice to give himself a means of amenity. By condensing his Mana into the dice, Hexipolia granted it the power to form a dimensional illusion. By rolling it, the dice opened up a fantasy world where he could find some peace. The Lu Estrona Dice was since passed down to the successors of his legacy, only to disappear when the ancient civilization collapsed, until it was finally found by Esker.
