I was looking back over demon/fel lore and something is sticking out to me. I was re-reviewing the lore and came to a strange realization, it’s a bit long-winded, so I’ll recap before I get to my thoughts.
[The Recap]
So cosmologically speaking, the Twisting Nether comes from the eternal clash between Light and Void, essentially a parallel to the Great Dark Beyond in the astral plane between worlds.
In this chaotic space, fel is clearly prominent, as this byproduct space is used as the spawn point for demons, the place where their souls go when they die, with them only truly being killed if they die either there or in a location of extreme fel energy.
Now, a demon in Warcraft lore, is merely a being that has consumed enough fel energy that their soul now redirects back to the Twisting Nether as opposed to the Shadowlands or any other similar realm. We can see this with beings such as Illidan Stormrage and Archimonde.
While most demons and demonic races that we see now are a direct result of the Burning Legion and its endless crusade, we know that demons were being formed long before Sargeras turned from the Pantheon and began recruiting and/or subjugating races into the Legion’s ranks, as dealing with threats such as these older demons was his primary task as a member of the Pantheon.
[The Theory]
So we know that many worlds seemingly just exist within the Twisting Nether as it is merely a part of the Warcraft Universe’s space that runs as the effective opposite half to the less chaotic Great Dark Beyond.
We additionally know that an excess of fel magic can work as a radiation, poisoning and changing the forms of those who remain in close contact with it without some level of magical protection.
Could it be that the source of these endless hoards of demons that Sargeras was tasked with disposing of were actually the result of countless planets being affected by the fel radiation that borders them?
As for why planets such as Argus and Draenor weren’t naturally corrupted, it has been stated that Argus was on the border of the Great Dark Beyond and the Twisting Nether, likely serving as a source of immense power while mitigating the naturally corruptive powers that the Nether brings.
It could also be argued that the Great Dark Beyond leans more towards Order, given connections between it and the Caverns of Time, along with it being traversed by the Titans frequently, though this is highly speculative.
On the topic of Draenor, it seems to have been implied a few different times that the Nether’s connection and prominence in Draenor was due to the planets destruction as a result of the Legion portals all being opened by Ner’zhul.