Sorcery Keystone Concept: Eldritch Study

I think Sorcery could use a fourth keystone, especially one focused around spellcasting. Here's one I came up with: Eldritch Study. The fantasy here is a utility/scaling rune that lets you practice casting your spells, becoming better and better at them. Once you master that spell, it becomes empowered.

Eldritch Study

• For every 100 mana you spend on an ability, that ability gains a Practice stack, up to 10. Abilities with 10 Practice stacks are considered Mastered.
• Abilities with Practice stacks gain 1% cooldown reduction per stack (counted separately from normal CDR, like Ultimate Hunter), and 1% per stack of the mana spent on them is refunded.
• Whenever you cast a Mastered ability, you gain a shield equal to 20-60 + (30% bAD) + (20% AP) which decays over 5 seconds after that ability's cast/channel time. Subsequent casts refresh the shield (they don't stack). Other allies affected by that ability also gain this shield. Enemies affected by that ability take magic damage equal to 10-40 + (15% bAD) + (10% AP) over 1 second. (A single cast of a spell can only apply these effects every 4 seconds).

As you cast spells, those spells get a small amount of CDR and slightly cheaper to cast. Once you've spent 1000 mana on any one spell, that spell will start granting you a shield for casting it, plus either granting a shield to allies you are casting it on, or applying a small burn to enemies you hit. The numbers seem small, but keep in mind that they are guaranteed to occur on every target affected, and there is no cooldown for separate casts. Compare to Summon Aery, which can only hit one target at a time, and can't affect you. AoE spells like Senna's ult or champions with several spells that can be used in tandem like Ezreal could generate a huge amount of value once their kit is Mastered.

This rune would be a good option on spellcasters with restrictive mana costs like Kassadin, Senna, and Ryze, as well as champions who like to spam their spells like Lux, Brand, and Ezreal. Since it stacks based on mana spent, not casts, it can take quite a while to Master a spell with a low mana cost. Spells with long cooldowns tend to have higher mana costs, so stacking them won't take forever. The rune wouldn't provide any real combat benefit until about 15 minutes into the game, rewarding players who stay safe with more frequent and potent spells in the mid-late game.