Personally, I'd wait before weighing in on this particular discussion, but there are few inaccuracies that will make you reconsider your initial theory, Firegriff
In the current storyline, Nissa discovered
Nicol Bolas corrupted Amonkhet, rather than create it. This occurred within the last few decades.
The Eldrazi titans were initially drawn from the Blind Eternities by Ugin,
Sorin Markov and Nahiri thousands of years ago and imprisoned on Zendikar, only to have been released when Chandra was lured to the plane.
We know from the lore, through Ugin and Emrakul, that the Eldrazi Titans are ancient beings that transcend time and space, they are in essence, entropy. Either way, the gate watch used the leylines on Zendikar to destroy two of the Eldrazi Titans, and Emrakul was drawn to Innistrad by Nahiri through the
Drownyard Temple. Emrakul subsequently imprisoned herself in Innistrad's moon, using Tamiyo's scroll.
I think Nissa is reminded of Emrakul, because of how helpless Emrakul made her feel when she took control of her body. In the same way she is made to feel helpless in Amonkhet, not being able to commune with the plane or to see through Kefnet's illusionary trap.
Without the entire of the current storyline having been released, I can make a vague inference. I think that
Nicol Bolas is draining the mana from Amonkhet to fuel his desire to become what he once was. Nissa sees a hundred, and a thousand worlds wrapped with a black shadowy line.
Nicol Bolas is no doubt doing the same to other worlds. Since the Gods of Amonkhet are essentially infused with the mana of the plane,
Nicol Bolas is no doubt using this connection to manipulate their essence, like Nissa does to Kefnet during the
trial of knowledge, only on a greater scale. Oketra has no knowledge of before the God Pharaoh when questioned by Nissa. It is likely that
Nicol Bolas had her memory erased.
There are eight gods of Amonkhet, right! However, the missing three could have been imprisoned by
Nicol Bolas, perhaps because they could not be manipulated in the same way the others were. (I can only speculate)