If you look closely at Demise's sword there is the symbol of the Triforce on it. Not only that, but it's upside down! What do you think this means? I think it means that Demise is is the complete opposite of good and that he is pure evil. What are your thoughts?
I made a big post on this. But, basically. I think Demise's sword comes from how the Master Sword becomes empowered with light in TP, and that creates a kind of shadow within the same Master Sword. This Master Sword then sits in the grove at the base of the Tower of the Gods (the other two timeline swords appear to be sleeping forever).
Well, the end of the child timeline of TP is Four Swords Adventures, which ends with a battle at the Tower of Winds (the Gods) which collapses. One of the major villains is Shadow Link, whom Ganon created using, basically the mirror of Twilight or a version of it.
So I think at the end of 4SA, Shadow Link gets the TP Master Sword, which has a shadow Master Sword within it. It's a bit more complicated how that gets back in time, but basically the running theory is child timeline Shadow Link merges with Downfall Timeline's AoL weakened spirit Ganon to steal the AoL Triforce and wish to go back in time.
This is Phantom Ganon, who becomes Demise. My theory.
There is some spoilery, but not really important evidence in TotK that Phantom Ganon might have existed in the ancient past. Like, before the first Ganondorf. I won't describe the evidence, it's minor anyway, but it's not like it's crazy to think this is possible.