I think it's OP.
Let's examine:
Rain of Fire 7
| Firestorm 5
This is not instant like firestorm, but you can activate it with an immolation or something else. So you could reduce the cost by 1, bringing it to 6|4.
I was reducing the cost by 2, but then i considered that there is no way to prevent its activation, except for basilisk blood (and lobotomize, i guess).
It can also be triggered the same turn the creature comes into play, so if you decide to use it, the opponent can't do anything to avoid it.
But it's is also a creature: an 1|3 should cost approximately 1
, while a 5|3 should cost something like 4
(3
because it's upped, see Mummy).
So, the final cost is
unupped: 6 for storm effect + 1 for creature = 7
upped: 4 for storm effect + 3 for creature = 7
Please note that this is my analysis, and i might be wrong. However i tried to be as accurate as possible.
On the art: i really like it, but i don't like the fading of the fire.