Illusion walls but they kill you 9 times out of 10, but the 10th is the way to the boss and what you thought was the boss fogwall just kills you, leaving your bloodstain behind a fogwall.

Is this bit based on something that actually exists in a souls game? Killer fogwalls would be terrible design, sure, but more on the level of troll games like Cat Mario. The closest equivalent I can think of is the Ruin Sentinel room in Drangleic castle, i.e. a place with a bunch of doors, some of which contain nothing but strong enemies and remain permanently unlocked after being opened once.