In Messmer’s castle, I think most of the NPCs as well as notes you pick up refer to both of the two required vanilla bosses’ corpses being moved or otherwise manipulated. They also talk about a ritual, I believe one NPC even connecting the dots for the player and saying explicitly what the ritual is for and that the corpse is for the ritual. It’s the most coherent and opaque FromSoft lore has ever been.
Whether it’s GOOD that that’s the boss is up for debate, I certainly don’t agree. But it isn’t an arbitrary decision.
Redmane Freyja and Sir Ansbach. You bring a letter to Freyja from Ansbach who elaborates some. You also bring a scroll about the ritual to Ansbach who lays it all out quite clear. The dialogue is all on fextralife so you don’t have to replay the entire DLC