So after a lot of thought, I believe I've come up with a pretty damn good theory.
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So in Der Eisendrache we have the achievement, "My Brothers Keeper". Where we actually defeat one of the "Corrupted Keepers" in order to obtain the soul of "Tank" 1.0.
I'm gonna go out on a limb here and say, "In DLC 2 on the Island, we will indeed defeat/kill another "Corrupted Keeper".
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If this ends up being the case, in DLC 3 & 4 were going to be defeating/ Killing 2 more "Corrupted Keepers".
Which I believe will actually turn out to be, the way the NO4 were seeing themselves in this different "Dimension". (Possibly, if they are in fact the Ancients)
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In Shadows of Evil we "Summoned 4 Keepers", to help defeat the Apothicons. Now with the NO4, were going back and defeating/killing these same 4 "Keepers".
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In my opinion, the Summoning Key will act in a similar fashion to the "Nav Card Tables". Once the DLC season is finished, we will take the now "Fully Charged Summoning Key" back to Shadows.
This time however, theres going to be a different Quest. The Quest in which we realize we've been the "Apothicons Servants" this entire time. This is when we will be able to do the "Upgrade Quest".
The ending will obviously lead into Black Ops IV.
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One other thing I want to add, since I feel this is the most relevant thread for it. Ive just completed D.E. Main Quest, so of course I was fascinated when it first happened.
I began looking at the Moon for the most part & I couldn't help but notice, is it just me or does there seem to be an "Apothicon Symbol" in the dead center formed from the debris?!?!