The body in cistern intrance?

[Hunter of Shadows]

- If you go to the cistern first, water horror won't be there and the room won't be engulfed by the shadown either after turning all the valves.

That's not a secret, it's an entirely random event

I thought this was already determined... this only happens if sanity is low enough

Yes, it's if your sanity is low enough, but even then it's still random, there's a random chance it'll happen or it won't