In the future, a powerful new species known as "shadow cyborgs" emerges from the fusion of humans, technology, and black metal. The government forms the Shadow Security Force (S.E.F) in each sector, all controlled by the Headquarters in Century City. This seemingly perfect system collapses when the Headquarters is attacked by the Dark Clan.

CORE, a senior cyborg soldier, discovers the plot too late. The Lord Commander of S.E.F is hacked, leading him to slay his own soldiers and replace them with a new generation of shadow cyborgs to overthrow the government. After the attack, CORE joins the rebellion, seeking revenge for his fallen comrades and aiming to dismantle the Dark Clan.

ShadowDiary is a hybrid of hack 'n' slash and action platformer genres, featuring a variety of weapons such as swords, buster swords, spears, and twin daggers, each with unique built-in combos. Starting from the Dark Forest, players will face dangerous enemies and traps, using their skills to evade attacks and execute extraordinary combos.