If you want demons, look no further than Berserk. Each one is characterized by something similar, however: They all came into being by sacrificing something that defined their former humanity - something that, when relinquished, was like abandoning a part of their soul.
I think this kind of creation would make for a really nasty character, but would it be too hard for Riot to pull off?
I think Riot seems to be more basing their demons off of living traits/qualities/sins. Aatrox is essentially living rage/war. Rhaast is essentially living malice/wrath. Eve is essentially living lust/anguish. Tahm is essentially living gluttony. They seem to be more focused on demons that are a bad thing personified.
This cannot work in the lore or League. The demons, or at least Evelynn, was born of primordial shadow that coalesced into mild sentience. Through the Rune Wars, she got a taste for the agony of mortals and thus now hunts them down for more.
The demons of this world were never humans, and I severely doubt humans could ever become one, considering that they probably all were primordial shadow at some point, even Kench. Therefore, this style of demon you speak of isn't possible.
However, Malzahar has a similar relationship to the Void, so this concept already exists in the game.
theres also the void beings like cho gath, kog maw etc i think they kinda classify as demons
