From an outsidetheuniverse perspective , the whole rain scene was an effing train wreck . There ' s nothing about it that even vaguely conforms to what we know about the HPU and was purely a MacGuffin for ending the movie with as little effort as possible , continuity be damned . From a let ' s d o d a m a g e c o n t r o l a n d w o rkthisoutandbedamnedforit perspective , the rain would have to be ineffective against all magic users . The mechanism for the lethe rain is different from the obliviate curse so the rules that apply to obliviation don ' t apply to lethe rain therefore it doesn ' t work on magic users , but on muggles it works if it ' s ingested drank or vaporized boiled or through physical contact . In order to make everything that happens during the lethe rain scene possible , I would also have to say that the magical component of the lethe rain taintstravels faster than the water it ' s mixed with . So for example let ' s say you ' re at one end of a pool with your hand in the water , and I ' m at the other end with a gallon of lethe water . I pour my lethe water into the pool and , before that lethe water can sufficiently mix with the pool water , the magic is already at your hand and you ' re affected by it . Porpentina ' s death chamber is another train wreck of sorts , leaving a lot of questions with no answers . The very fact that Porpentina was arrested and sent for execution in a matter of seconds , with no trial or hearing or anything , leads me to believe that the whole thing was a setup by Graves and people who were . sympathetic to his cause . But that ' s another discussion . As for the chamber itself , assuming the chamber didn ' t malfunction during the escape which is what I assumed , it ' s likely that the mechanism is sort of like the Mirror of Erised where the victim becomes so entranced by his or her own memories that heshe doesn ' t even recognize the danger even as it ' s happening . In that way the chamber could be seen as a more humane option than , say , avada kedavra or Dementors .