New Sorcery Keystone: Soul Drain

We need a new sorcery keystone for damage over time like conqueror, but for AP.

Battlemages are a bit like Bruisers in that they do(past tense for bruisers) not have a keystone that makes sense for them. I'm thinking stuff like Karthus and Swain right now.

Here is my suggestion for it:

Dealing damage to champions with abilities grants mana. Once XXX mana has been gained from a champion this way in a short amount of time, the champion becomes Drained for X seconds, losing 10% of their Magic Resist and refreshing on further ability damage. Gain XX AP per champion that is Drained.

So it's really really good if you get 3-5 champions in your sustained aoe damage, but meh otherwise. Unlike Conqueror, you don't get the full effects in a 1v1 nor true damage, and takes time to charge up on each champion (again, unlike conqueror). Not super useful in laning phase though...at least compared to other keystones.

I think it could be a good keystone for the game

EDIT: changed the sustain to mana gain (doesn't do a whole lot but it helps and keeps it in line with the soul draining theme)

Also, it works a bit like press the attack, in that once the rune places the debuff it doesn't do any more damage nor grant more mana.

https://media.lolusercontent.com/api/embedly/1/image/resize?url=https%3A%2F%2Fpreview.ibb.co%2FeO0Ufe%2FSoul_Drain.png&key=f0abbd34f14549f3a15cd94dd9970851&width=150