i had a similar mechanic for bunnify effect, was called radiation and was reduced by 50% each turn.
as far as balancing goes, this one is tricky because it is triggered by a spell. if you save up 5 of these in a stall deck, it does 15 topend damage, for 25 quanta, thats a terrible ratio, but it is uncounterable, unless purify can stop it. still a dragon would do more damage over a few turns for half he quanta. if anything its pretty UP, and even if it were balanced, its something fire doesnt need to add to its arsenal, firebolt pretty well covers the cc or damage your opponent, and waiting to use it makes it more powerful, niche.
the card name is already taken by the upgraded rain of fire.