Reminiscent of Torch Song. Grows faster if the opponent's playing more spells (or in multiplayer); grows less if they're ahead on the board already. Could be cheaper.

Kinda usable like armageddon clock, only more focussed. But then, it grows linearly instead of exponentially-with-backoff.

I don't like that once it's grown it can't be stopped - they try to kill it, and in response it grows and explodes. Not sure what to suggest for that.

Thank you. That sounds about right. I wasn't sure about costing; I still cost things too cheap more often, so I often err on the other side with effects that don't absolutely desire power. It's probably not much stronger played earlier; indeed, from that perspective, maybe it should be "Leyline of Kaboom" :)

Added mana to activation: if you're playing this you should rarely not have 1 mana open, but does make it slightly more situational because you opponent MIGHT have artifact removal other than krosan grip. Compensatarily reduced cost from 4 to 3.