Well, considering the original 1.0 incarnation of Thaumaturgy used blood rites and self-sacrifice to cast spells, I had always assumed these blood rites were tied to ancient Thaumaturgy, like that of the first Magi in the early Fifth Astral Era, pre-Shatotto. So not necessarily "Void Magic" but just ancient Thaumaturgy, that was later used to unlock the Void during the War of the Magi using Black Magic rituals.
As for why Demon Walls are in Amdapor, the city of White Mages, remember that White Magic wasn't gifted to Mankind until after the discovery of Black Magic, but that the Magi of Amdapor existed long before that discovery was made. So if they were Magi, and Conjury wouldn't exist for another 2,000 years, what were they Magi of? Likely also Thaumaturgy. Just instead of taking this ancient form of Thaumaturgy down the route of Black Magic, they focused more on Thaumaturgy's healing properties over its destructive properties like the Black Mages ended up focusing on.
This is in keeping with early 1.0 THM being more than a respectable healer and having HP/MP costs with all of its spells, and even a spell called Blood Rite.
If you also wanna look really deep into it, Thaumaturgical arms use staves often crafted from bone or aetherially conducting metals such as silver or gold. But it's said that bone channels particularly well. This sounds very much like it'd be something from ancient times. Bones and Blood to cast magic instead of silver and gemstones like we use today.
Further down this train of thought, consider how the ancient peoples of Eorzea believed in blood letting and chigoe sucking to release bad humors or aether from the body. (Until they later discovered chigoes carried diseases of their own - like Blight.) What if... way way at the start of the 5th Astral Era when Thaumaturgy was first being discovered and explored, the early Magi weren't sure what "aether" was? What if, in an effort to tap into their inner reserves of aether to cast spells, they externally sacrificed blood or blood let, to invoke magic?
The principles of Thaumaturgy have always been limited by one's own aether, just as one's blood and bones are limited as well. Perhaps early magi used their own blood and the bones of another or recently deceased to make their spells. This would require incredibly more prep-work, resulting in these elaborate glyphs and rites drawn in blood that are referenced in lore.
Just a thought/theory.
But still would say these ancient "blood magic" practices often referred to in lore are actually just the early incarnations of Thaumaturgy.
As for why Demon Walls are in Amdapor, the city of White Mages, remember that White Magic wasn't gifted to Mankind until after the discovery of Black Magic, but that the Magi of Amdapor existed long before that discovery was made. So if they were Magi, and Conjury wouldn't exist for another 2,000 years, what were they Magi of? Likely also Thaumaturgy. Just instead of taking this ancient form of Thaumaturgy down the route of Black Magic, they focused more on Thaumaturgy's healing properties over its destructive properties like the Black Mages ended up focusing on.
This is in keeping with early 1.0 THM being more than a respectable healer and having HP/MP costs with all of its spells, and even a spell called Blood Rite.
If you also wanna look really deep into it, Thaumaturgical arms use staves often crafted from bone or aetherially conducting metals such as silver or gold. But it's said that bone channels particularly well. This sounds very much like it'd be something from ancient times. Bones and Blood to cast magic instead of silver and gemstones like we use today.
Further down this train of thought, consider how the ancient peoples of Eorzea believed in blood letting and chigoe sucking to release bad humors or aether from the body. (Until they later discovered chigoes carried diseases of their own - like Blight.) What if... way way at the start of the 5th Astral Era when Thaumaturgy was first being discovered and explored, the early Magi weren't sure what "aether" was? What if, in an effort to tap into their inner reserves of aether to cast spells, they externally sacrificed blood or blood let, to invoke magic?
The principles of Thaumaturgy have always been limited by one's own aether, just as one's blood and bones are limited as well. Perhaps early magi used their own blood and the bones of another or recently deceased to make their spells. This would require incredibly more prep-work, resulting in these elaborate glyphs and rites drawn in blood that are referenced in lore.
Just a thought/theory.
But still would say these ancient "blood magic" practices often referred to in lore are actually just the early incarnations of Thaumaturgy.
