[Suggestion] Sorcery Rune for periodic magic damage dealers

Currently there really isn't a good rune choice for magic damage dealers that deal a continuous amount of damage over time. The "1,2,3" Runes are ok but they're more made for burst damage than actual DoT damage dealers. The closest we get to an ok rune for them is Summon Aery, but that is a more support focus rune as it provides bonus to shielding and it base damage is kind of low. These champions can do good damage but there really isn't rune that works well with their method of dealing damage. The types of champions I'm talking about are as follows: (to some extent)

So first thing I'd suggest we move Summon Aery to the_ Inspiration_ runes to make room for a periodic magic damage dealing rune. The Rune I suggest would require ability-based damage to take place (sorry no auto attacks, otherwise it would OP), and be dealt frequently. Similar to the old Deathfire Touch, it would apply a dot from dealing damage with your abilities; here have a look:

Grimoire of 1000 Plagues

Upon dealing any ability damage (direct or periodic) apply plague to the target champion or monster dealing 1(level 6)/3 (level 11) (/5 level 16) (based on level)(+5% of Ability Power or Bonus Attack Damage) magic damage over 4 seconds. Damaging the target champion already afflicted a plague will refresh the duration and add 1 more plague stack to the target, stacking up to 1000 times.

So the idea here is if you do alot of frequent but small hits of damage from your abilities it can result in apply a rather painful amount of plagues to the target. Obviously its unlikely anyone will get the stacks up to 1000 especially before a champion dies, that number is there for fun. I tested out the damage projections using our good friend . You see his E Toxic shot would apply plague stacks both from it's initial hit and all the periodic hits, meaning its 5 plagues per auto. In a realistic scenario with Teemo having about 240 AP at mid-game at level 11 I found that with 2 autos, his Q, an his ult he can get 15 stacks garentee'd. This means the Rune Grimoire of 1000 Plagues will deal 45(+12 AP) every second over 4 seconds or about 228 extra true damage. Not a bad haul at that point in the game, and when he buy things like or (which this item makes everything crazy) you'll see his Plague dots become even stronger. The Reason this rune won't proc from autos is because it would become a serious balance issue and on some champions like it would be utterly broken; also the rune is for mages first not auto attackers.

EDIT: changed to magic damage from true damage; Its a reasonable change as the idea isn't meant to melt tanks.