I like the idea. It allows you to take your opponent's advantage and turn it against him. It takes 3 turns to 'pop', can be stolen, deflaged so that the effect does not happen, and has a skill curve into when to play it. Too early and the effect is not used due to the low damage from the opponent, too late and you won't live to see it go off. It can also be used to detour your opponent from killing you for a bit, allowing you to catch up.
Healing would be powerful with it, but I don't think too powerful. Healing just allows you to live longer to see the damage reflected. The damage must still be dealt to you, and if you can heal through it, then this just allows you to benefit from those heals even more.
It might need some re-wording. Something like "After 3 turns, deals half of the damage you received to your opponent and Karma is destroyed."
As for the element it falls under, Any element could use it, I see darkness as a good candidate due to what OldTrees said and Entropy due to it already having such effects like antimatter. Maybe even time. Life fits but is not ideal, life doesn't deal direct damage through spells, it uses creatures or poison, very earthy, alive things. Karma is more of an idea that kills.
