It doesn't fit if you start the ability on a new line, as you always should. In addition to the suggested card text being confusing and generally unaesthetic, the ability you suggested, as written, tries to make use of "multi-targeting", a mechanic that would throw the card into the "can never be implemented" zone.
Full-field quint strategies implementing this card are temporary because cloak is temporary. Invisibility and other ways to make cloak disappear have nothing to do with the fact that this card is not "making everything on the field immaterial for the cost of a 1 card sac", as you claimed, and already has a "limited duration", as you suggested.
... only major uses I see so far are mulit-color decks for pandamonium and unstable gas... maybe flooding as well
And this statement makes me question whether or not you actually understood the card before typing all those criticisms.
You can't target Pandemonium with this ability, and using the ability on Unstable Gas has no effect because Unstable Gas doesn't affect anything.
Alright, but this does fit: "Weapon: Deal 7 damage each round.
: Sacrifice a card, make N slots immortal, N=card cost"
Also, I did understand the original meaning, I.e. changing an ability so that it makes things it used to target immaterial rather than its original effect. I think that steps on Aether's toes a bit to much as is, which is why I was suggesting an alternative.
Lastly, I understand that you cant "target" pandemonium, but pandemonium is one of the few cards that targets everything on both sides... like unstable gas. So having the ability to "quint" your creatures helps there. And actually, unstable gas technically targets all creatures in play:"
(sacrifice card) inflict 20 damage to your opponent and 1 damage to all the creatures on the field"
In other words it targets every creature on the field and does 1 damage to them. So it would be a mass quint for 1
under the current ability here... I think thats a bit unbalanced myself.
Also, if the N slots are randomly chosen this is not a multi-target spell, so it could be implemented.