Akki (Japanese Word for many things... Demon, Evil... etc.) Fic's are Storylines that give off an aura of Darkness on normally happy settings.
Fanfiction has many examples of Akki Fictions....
Say there is a fanfiction story for naruto... but in this he is a necromancer.
He Is completely Evil... near insane... these stories are Called Akki (Story/Fiction)- (Akki Fics)
There are fic's on fanfiction.net that are better examples of this-
Naruto the Dark_Lord
Black Flames Dance In The Wind Rise of Naruto