Vanilla Creatures with X|5 stats (X varies between creatures) tend to have a Cost of X quanta when unupped.
So a 3|5 creature would be worth 3
. (although it would not be a Fire creature because Fire has high attack low hp usually)
The backlash effect would need to be balanced with above average stats.
It probably should remain brittle so that the effect occurs more often.
Suggestion (mostly guesswork)
Casting Cost: 4
|3
Attack: 6|7
HP: 1
Effect: When destroyed deal controller 5 damage and all of controller's creature take 2 damage.
Notes on suggestion:
I increased the cost so it would not compete with Ash Eater
I increase the Attack to be worthy of the drawback
I simplified the drawback and decreased the severity
I do not know if my suggestion is balanced but it appears to be closer to balanced and fit the idea better for ETG.