The Army of Darkness is an army, largely made up of demons and other hellish entities, created by Shinnok during his banishment in the Netherrealm.
After his escape from the Netherrealm with the help of the demon sorcerer Quan Chi, Shinnok used this Army of Darkness to invade Edenia. However, Raiden and the Earthrealm warriors were able to stop Shinnok's advance, and his army was eventually defeated.
The unholy army of Shinnok can be seen in the official Mortal Kombat 4 comic book where some of the demons tried to kill Fujin, who was saved by Raiden, Liu Kang and Kai.
The Army of Darkness would later make their first appearance in a game in Mortal Kombat X's Story Mode, where they are seen causing destruction to Earthrealm as its armed forces try to stop their attack. Shinnok's army would also attempt seizing the Sky Temple so that Shinnok could access the Jinsei Chamber and therefore corrupt it, but they were eventually defeated as Shinnok was imprisoned within his own Amulet.
An Oni, presumably a member of Shinnok's army, also makes a playable appearance in the mobile version of Mortal Kombat X.
Shinnok's Uprising[]
The Army of Darkness escape the Netherrealm and invade the Edenian Palace as the enchanted opal gifted to Queen Sindel shattered into a portal! The denizens didn't stand a chance against it! Some of the Onis assaulted Fujin as they followed him falling into China while Shinnok attacked the heavens, till Liu Kang, Kai, Kung Lao, Nightwolf and Aiko destroyed them.
It wasn't over as they continued to attack the heroes, causing them to separate throughout Edenia! Through the endless hordes summoned by Quan Chi and Shinnok, they needed to recruit the cavalry consisting of the Tarkatans (who broke their cover), Shokan, Centaurs, and Saurians. After the fall of both nemeses, they never returned since as they kept from being summoned from the Netherrealm.