I think it happened in phases. Some other people earlier have wrote better timelines, but mine is
-Byrgenwerth finds Yharnam the white queen in the old chalice dungeons.
-They start experimenting with blood ministration and it's all good times for a while.
-First problems with blood appear. The church is formed.
-The church finds the slugs in the dungeons. The church takes them over and eventually summon Ebrietas from them.
-Blood Beasts appear. The church creates the hunters.
-Ebrietas and the Celestial Emissary create their own wing, the Choir.
-Mensis, probably an offshot from Byrgenwerth, discovers that the Great Ones inhabit the dream dimension by default.
-Someone -probably Kosmos, Ebrietas' father- transforms Rom, some Byrgenwerth student, into the Vacuous Spider.
-The Vacuous spider has the power to create a veil to keep the Great ones hidden from the populace.
-The church closes Byrgenwerth.
-Using the veil, Mensis and the Choir stop being reluctant over their experiments and start bringing more and more Great Ones from the dream dimension, since they will be kept hidden from the people.
-the game.
I don't think so; only hunters get to go to the hunter dream, and the people who go to the nightmare frontier do not seem anchored to that place like your hunter.
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