Ultimately I suggest this:
"Hey guys, for tonight's adventure I need you all to be put to sleep with no save or resistance. If you agree to let it happen, I will say it will make a better session in my opinion. Any objections?"
Screwing over players who invested in defenses against a thing by pointing to an obscure rule or handwave or broken magic item is basically just being an asshole. If the players would object... then don't do it. Either let the dice fall where they may or just discard the plotline. A good game trumps any mechanical concerns and good games are built on trust and openness.
"Hey guys, for tonight's adventure I need you all to be put to sleep with no save or resistance. If you agree to let it happen, I will say it will make a better session in my opinion. Any objections?"
Screwing over players who invested in defenses against a thing by pointing to an obscure rule or handwave or broken magic item is basically just being an asshole. If the players would object... then don't do it. Either let the dice fall where they may or just discard the plotline. A good game trumps any mechanical concerns and good games are built on trust and openness.