The Army of Darkness is a a massive legion of demons and other monsters and dark entities created by the fallen Elder God Shinnok in his quest to rule all the realms and take his revenge on his former allies, the Elder Gods. They are mention in both Mortal Kombat 4 and Mortal Kombat Gold. They also appeared in the official Mortal Kombat 4 comic book.
During his banishment in the Netherrealm, the fallen Elder God Shinnok created an army, largely made up of demons and other hellish beings to serve him.
After his escape from the Netherrealm with the enlisted help of the sorcerer Quan Chi and the traitor Tanya, Shinnok used this unholy force of evil to invade Princess Kitana's peaceful kingdom of Edenia. However, the thunder god Raiden and the heroic warriors of Mortal Kombat were able to stop Shinnok's advance, and his army was eventually defeated.