Information on the Rapper:
Dr. Edward Richtofen is a playable character in the Zombies mode within the larger Call of Duty series. He is introduced as a stereotypical sadistic Nazi scientist who works together with soldiers from other WWII factions, Tank Dempsey, Nikolai Belinski and Takeo Masaki, to fight the undead. Richtofen is later revealed through background documents to have caused the zombie outbreak, effectively making him the main antagonist, and indeed, in the final Zombies level of Black Ops taking place on the Moon, he betrays the team and takes control of the zombies through stealing the body of previous controlling host Samantha Maxis.
In the Zombies story of Black Ops II, Richtofen is the main villain controlling the zombie hordes on the ruined Earth, and the new survivors are given conflicting mental instructions by both him and his seemingly more ethical rival and colleague in creating the zombies, Ludvig Maxis. These events culminate in an underground stage where, if siding with Maxis, it is revealed that he is just as bad as if not worse than Richtofen and will now soon destroy the Earth completely, and things get no better either if Edward succeeds.
This is the bleak chronological end of the CoD Zombies storyline, but there is one more level released afterwards taking place before all others, at the end of which it is finally revealed that Richtofen and Samantha are two mere children playing with action figures, and the entire story was just in their imaginations.
(Lines in italics are said together with the other three zombie-slayers)
The Doc is in, with schemes aplenty and an ace in every hole;
When they all come to fruition I'll assume direct control!
...Call it Normandie der Toten!
Time to cause some Grief!
From Shangri-La up to the Moon, we'll take the carnage anywhere!
...We'll crack your skulls like Easter eggs!
This zombie-slaying team's as lethal as your whole Collector Base!