How to balance/rework Deathfire Grasp and bring it back to Summoner's Rift

As the title says...

:

Stats remain the same

Active changed to passive:

Landing a ability on an enemy champion deals 12% of the target's maximum health in true damage; afterwards, increases all subsequent magic damage by 40% for 8 seconds. (60 second cooldown)

While the new may seem a bit over-powered, it makes up for it for having a fairly long cooldown.