5 
for instant death of opponent's next creature and opponent takes 15 damage. Is this balanced?
Who said it had to be their creature? It triggers on your creatures as well. So you can set it off with your own creature.
Also, consider Unstable Gas. It is one card that deals 20 damage and the equivalent of a Thunderstorm to both sides.
This requires requires two cards to work and only deals 10 | 15 damage. And unless you play a creature, there is no guarantee it will ever go off.
It still seems a little too potent to me. The damage isn't too bad compared to Unstable Gas, but instant kills are typically given very high weight.
I don't have an issue with it zarking the next creature that gets played, but an auto-kill seems a little too much.
A better way would be to have it deal damage to the creature rather than just outright kill it.
One very amusing implementation here would be to have it deal a large amount of damage that "spills over" if the creature dies. This gives players an incentive to use / sacrifice a high HP creature rather than just using their most worthless one as fodder.
E.g.
"Your opponent gets this card.
Detonate: Deal 15 damage to the next creature played. If it dies, deal 2 x excess to owner"
This would help make high HP creatures much more valuable in the metagame. Which would certainly be beneficial to
and
This is just an example, you could tweak the damage dealt and spill over factor to help balance it properly.
