Two quests in one
The main quest of Shadows of Evil was first known as “Playing For Keeps”. It comes in two parts. Apocalypse Ascendant is the five rituals that open Pack-a-Punch and free the giant Apothicon, with the achievement The Beginning of the End. Apocalypse Averted turns the crew against the Shadowman. It gives no achievement, but a Dark Ops calling card and the Summoning Key icon on the map selection screen, one of the marks needed for the super Easter egg of Revelations.
Up to the boss fight, the quest can be done with any number of players. The last step and the ending cutscene only trigger with four players.
