Immortal Armies are armies of the living dead sustained by the magic of Cup of Life. Only two such armies have been created in the history of Albion, the first by a great warlord and the second by the High Priestess Morgause.
History
Warlord's Army
Hundreds of years before Uther's reign, the Cup of Life fell into the possession of a warlord who took a drop of blood from each of his men and collected it in the Cup. Such was the its power that they were made immortal where they stood.
According to Gaius, the carnage wrought by this army was beyond all imagining. It was only defeated when the Cup was emptied of the army's blood, which broke the enchantment and destroyed it instantly (The Coming of Arthur).
Cenred's Army
Morgause used the Cup of Life to make Cenred's army immortal. She took a drop of blood from each of his men and collected it in the Cup, then performed a ritual which made them not only immortal, but transformed them into the living dead. The ritual also allowed Morgause to take control of the army, and with no further use for Cenred, she ordered one of his men to kill him.
Under Morgause's command, the immortal army easily conquered Camelot and Morgana was crowned Queen. With Uther and the Knights of Camelot in prison, the immortal army was tasked with hunting down Arthur and his allies. They tracked Guinevere and Leon to Arthur's hideout in the Darkling Woods, but failed to capture the group due to Lancelot and Percival's intervention.
The immortal army later cornered Arthur and his allies in the castle dungeons when they tried to rescue Uther. They attempted to kill Merlin and Lancelot, who were on a private mission to find the Cup of Life, but they easily destroyed by the sword Excalibur, which had the power to kill the undead. After a brief skirmish with Morgause, Merlin emptied the Cup of the soldiers' blood and the army was instantly destroyed (The Coming of Arthur).