Touch of Death (Sorcery keystone for DOT mages)

I've honestly always felt like the keystone choices for damage-over-time mages is extremely poor. They can rarely proc Electrocute or Phase rush in a reliable way, aery is sub-par, especially since they're not usually low-ranged enough to retrieve it quickly, and they generally don't have enough cc on their spells to make comet useful.

With this in mind, I think a keystone such as this would really help dot mages (and other damage-over-time users) to have an impactful keystone option (Please note that I'm not great at balancing numbers, focus more on the ideas than the numbers).

Touch of Death: All spell damage applies a stack of "Touch of Death" for 6 seconds (Can only apply 1 stack every second), up to six stacks. Your spells deal 2-6 +(.03 ap) additional damage per stack that the target has. When 6 stacks are reached, all stacks are expunged, and the enemy is dealt 10% of their missing health over 4 seconds as magic damage, triggering an on-target cooldown of 20s. (in other words, if you get to 6 stacks, that's when the cd initiates, not if you don't make it all the way there).

This keystone would work well with champions such as malzahar, cassiopea, talliya, anivia and rumble.