At some point, a kind of aberrant creature called a Grudge suddenly appeared in human society. Investigations revealed that these beings were born when human negative emotions accumulated to a certain level. These emotions mainly include the seven: joy, anger, sorrow, happiness, envy, jealousy, and hatred. Feelings such as arrogance, conceit, haughtiness, pride, and fear can also give rise to negative emotions.
Once enough negative emotions have accumulated, they transform into energy and give birth to Grudges.
Since Grudges are born from human negative energy, they feed on humans to sustain their lives.
To deal with these sudden arrivals, human institutions trained people with unique talents and formed an Investigation Division dedicated to tracking down and hunting the Grudges scattered among humanity.
Grudges are classified as C-rank, B-rank, A-rank, S-rank, and SS-rank, with each rank higher up being more powerful.
Investigators, meanwhile, are ranked as Second-Class, First-Class, Senior-Class, and Special-Class Investigators.
In other words, only those who can fully handle C-rank Grudges and have the strength to deal with B-rank ones can become Second-Class Investigators and officially begin hunting Grudges.
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