Call of Duty Zombies mode has cultivated a large fanbase over the years. Debuting in 2008's Call of Duty: World at War, the mode has taken on a life of its own with a unique story and original set of characters. It has appeared in every Treyarch title and encompasses a total of 48 maps.
Some of these maps are remakes of one another, such as The Giant, which is a remake of World at War's Der Riese map. Nevertheless, that's a high amount of content with a lot of little details that the player may have missed over the years.
10 A Great War
Eons before the first Zombie broke down barricades on Nacht der Untoten, there was a massive conflict waged between a race of beings known as The Keepers and their corrupted brethren who later became known as Apothicons.
Though these two factions were once the same, the discovery of the Dark Aether tore them apart. The corrupted Keepers are eventually defeated, but in their loss, they set the stage for future events that shake the very multiverse itself.
9 Dr. Monty
Among the first beings to ever exist, Dr. Monty is an incredibly important yet mysterious character. As a keeper, he was very good friends with The Shadowman and even tried to save him from being corrupted by the Dark Aether, though in the end he just ended up becoming corrupted himself.
It is not clear if Monty is good or evil, but he does help the main characters in significant ways. For example, all of the wall weapons are a result of Monty's magic as is the mystery box. Though this seems to make him good, it is also possible that his motives are self-serving.
8 It's A Multiverse
There are a number of universes shown throughout the Zombies storyline. For example, the Alcatraz Island shown in Mob of the Dead is its own purgatory dimension. The Group, Primis (consisting of Dempsey, Takeo, Nikolai, and Richtofen) is an alternate universe equivalent to the main timeline's Ultimis crew (from the original World at War).
In addition, maps such as Shadows of evil, IX, and Dead of Night all take place in their own universes. The plot even revolves around the idea of fixing the corrupted multiverse.
7 Richtofen Is The Key
Possibly the most important figure other than Dr. Monty or The Shadowman, Edward Richtofen is responsible for much of the events in the Zombies storyline. He is the one who brought the various groups of characters together.
Richtofen was also the foremost scientist when it came to the research and application of element 115. Though his characterization varies depending on the universe, he is undoubtedly important to the overall scheme as it is his to begin with.
6 Element 115
This elusive source of power is where everything began in the Zombies universe. Completing the Easter eggs will reveal 115's significance to the story. 115 was used to create much of the advanced technology found in the game such as the wonder weapons, Pack a Punch machine, Perk machines, and more.
It is also what created the zombies themselves. Not much is known about where exactly element 115 comes from but the player will find a strange, electrified meteor sitting outside of the map Shi No Numa. It is implied that this is 115 in some form.
5 Agartha
Much of the story revolves around the idea of Agartha, or The Aether. The home of Dr. Monty and the other Keepers, this mysterious place is known as the first dimension. It has a counterpart dimension known as the Dark Aether as well.
More than one character has tried to gain control of this dimension (including Richtofen), with entire wars being fought in the process. Ultimately, it is sent into the Dark Aether in Black Ops 4 along with Dr. Monty himself.
4 Time Travel
Time travel is a major part of the Zombies storyline. This is first observed on Call of Duty: Black Ops' Kino der Toten map. Here, the main characters are thrust from the year 1945 to 1968. These characters would also be teleported to the present day for a short time on Call of the Dead.
Another Instance of this can be seen on Moon in which the players are teleported to the year 2025. This particular fact is revealed in Black Ops 4's Classified map as the Earth is still intact in 1968.
3 Samantha Maxis
Despite being a child during most of the Zombies storyline, Samantha has taken on an almost mythical status within the lore. She is the demonic voice players hear constantly through the game and is in control of the zombie horde. Samantha remains in control of Agartha for much of the story after her supposed death at the hands of Richtofen.
This does not last however as Richtofen eventually replaces her through his own machinations. She is also the only known survivor of the ending to the Aether storyline in Black Ops 4, appearing in Call of Duty: Black Ops Cold War's new timeline.
2 A New Beginning
After the conclusion of the Aether storyline, only one uncorrupted universe remained. It is in this streamlined universe that Call of Duty: Black Ops Cold War's storyline takes place. This means there is an entirely new history, set of characters and threat to be dealt with as a result.
Luckily, Samantha Maxis has survived the events of the previous story and assists against the threat of Omega Group. It is unknown if anything else survived the plunging of creation into the Dark Aether.
1 Omega Group
Much like group 935 in the old timeline, Omega Group is a faction hellbent on studying the Aether and applying its research for its own ends. This time, however, Omega Group is a state-funded organization and it exists as an extension of the Soviet Union itself.
Using unorthodox methods, Omega would attempt to forward the Soviet agenda around the world. With the discovery of the Projekt Endstation facility, Omega was able to open a gateway into the Dark Aether. They would then study it, and develop new technology as a result.