So multiple times people call Hades the Devil and he says he isn't, nor a demon. So does this mean the Devil (Lucifer/Satan) is real? Was he really an Angel and If so is God, along side Angels real in some realm. Could He rule the region people go to, the worse hell. Or is he simply saying that he isn't the Devil from the myths. Could he also have been some powerful magician that was banished to hell?
It's ambiguous because there were some Satanic vibes about Hades (his backstory, his lair etc.) but we also got minor hints of a more demonic presence in regards to the Worse Place/River of Lost Souls, with effigies of a Satanic/Chernabog-like figure above "Abandon all hope ye who enter" which refers to Dante's Inferno.
Some form of demon definitely exists in the Onceverse, and is comparable to deities (I believe Hades at one point says that someone who calls him a demon is "close").
I think it's interesting that Season 5 basically left room for the Land Without Magic to operate under different rules than the Fairy Tale Realms; we never saw any natural LWM inhabitants in the Underworld (i.e., Greg and/or Tamara). Season 7 killed that by making the Land Without Magic no different from any of the Fairy Tale Realms except in history, but up until that point, I think they were trying to leave room for the possibility of Land Without Magic residents having a different afterlife system, possibly the Christian one.
Hades someone implied the Devil existed, with things like "people are always conflating us". I always assumed they made Hades's Underworld more or less Purgatory, so the Better Place could be Heaven and the Worse Place could be Hell.