The new video features Game Director Tom Chilton and Principal Game Designer Kris Zierhut, and covers the whole design process. After letting the community know that there wouldn't be a new class with every single expansion, when it came time for Wrath of the Lich King, especially with all its game-changing content, it was time to finally offer a new class.
Now, of course, with this move, the new class had to be memorable and be worth the wait. Death Knights, being iconic and important to the lore of Northrend, were it. With much concentration of Arthas and telling his story in the expansion, the Death Knight just worked.
Drawing from several other plans and classes, including Runemaster, Necromancer, the classic Warcraft Death Knight, and came up with what Chilton calls “concentrated coolness”. Given the history of the class and why it has been celebrated as we get ever-closer to Wrath Classic dropping (with the pre-patch coming tomorrow, when Death Knight creation will also open), this is an appropriate phrase.
They needed Death Knights to feel authentic, and their overall design philosophy was to not water down the classes and make sure if they did this, that the Death Knight would feel different than the other armored magical classes. It would also potentially increase the number of tanks to a level that would be more sufficient, especially with the added distance you could pull from.
Another element they mention is that including the Death Knight also meant a redemption journey. Since you are changed by the Lich King and then you are later fighting against him, you also face a reckoning with your past from the regular people who your actions have affected. This is also why creating one is unlocked later. Animations, resources, and other considerations also had to be done with this process.
You can watch the full video on the creation process, and preview the trip down memory lane coming on September 26th when Wrath of the Lich King Classic is officially live.