The damage inflicted on the opponent just seems too much, no matter what the cost. Rather than making it dependent on the number of creatures, I recommend setting it at 1 damage for unupped and upped. That way, stacking doesn't get too powerful.
This is also a pretty good hard counter to adrenaline, and it enforces the Life/Death antagonism.
Also, is the recoil damage considered spell damage?
Some other options:
1) Make it only trigger once at the end of any turn you take damage (then stacking may be ok since it simply mirrors empathic bond)
2) Make it a % chance to trigger when you take damage (this is probably better off as a shield then)
3) Limit it to non-physical damage. (which would be quite unique)
At any rate, its needs to be tonned down in some way since it seems quite OP right now.
I do find death an odd choice... I would have thought it would fit under

or maybe

or

more.
All that aside, I like the overall idea. Once you get it more balanced, it could have some very interesting possibilities.
One last idea that would make it more

themed would be to have it trigger when the opponent recieves healing... It may still need to be toned down some, but it would provide a seriously potent anti-Heal mechanic.