As for instant kill cards:
Otyugh Devour and Maxwells' Paradox to name just two.
Rain of Fire can be considered a global 'instant kill all creatures' - seeing as a vast majority of creatures in the game are below 3hp.
I personally like this idea of 'kill creatures with same name' - a potential way of getting around Immortality? Eg. Your opponent has two Otyughs, one is Immortal and has 14/17 stats, the other is not immortal and has 2/5 stats... This ability, in my opinion, should effect creatures regardless of whether or not they are immortal, provided the same creature is in play to act as a target for this ability.
As for the cost, 7 quanta both upped and normal - and as akromat states: the main uses are for 'stalling' decks which use swarm and mob-rush tactics. Most notably Aflatoxin/Malignant Cells, Firefly Queen/Fireflies, and Boneyard/Skeletons. In the Firefly and Skeleton example, this ability does not go after the direct root cause of creating the creatures, but is a powerful way of getting rid of spawned creatures already in play. And as such, it is a lot weaker than Rain of Fire, to a certain extent.
If one was to use rain of fire to clear Fireflies, you lose 1 potential damage per firefly, if it was used to clear normal skeletons, then it is 2 damage wasted per skeleton. This amounts to a pretty ineffectual use of the Rain of Fire card - and who carries 6 thunderstorms?
I like it - but it is devastatingly powerful. I wish there was a mechanic for more dynamic ways to use quanta. Right now we have "Deal 3 damage + N damage; where N is your X Quanta divided by 5". Or more or less complexly, Dissipation Field which uses your quanta to negate damage.
If this card were to work like say, remove target creature for 7
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and every additional creature with that name for
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per creature. Then it would be a lot better balanced.
Just my thoughts.