Tier1 (1)
-Leaking Flask: Heals allies on its way for 35% of the healing.
-Leaking Vial: Blood Vial spreads blight on its path.
-Cannibalism: Whenever an enemy is killed near you, gain 1% health and mana. (5% on Hero kills)
-Conjurers Pursuit
Tier2 (4)
-Health Puddle: The Potion leaves a puddle behind for 5 seconds. Allied Heroes can click on it to heal [75 + 20 per level] over 3 seconds (1 time per Hero).
-Blight Overload: Cover a huge area around you with blight. 40 seconds cooldown.
-BlightTastic: Frenzied Heroes also spread blight with Basic Attacks.
-Protective Shield
Tier3 (7)
-Forsaken Army: Enemies killed on blight, return as ghouls. (minions = knight minion with 25% health,
merceneries = knight minion with 100% health, Heroes = knight mercenary with 50% health) Last until killed.
-Feedback: Decrease the cooldown of Toss Potion by 1 second for each Hero healed.
-Mad Rush: Increase the movement speed bonus to 30% for the first 2,5 seconds.
-Cleanse
Tier4 (10)
-Healing Vapor
-The New Plaque
Tier5 (13)
-Death and Decay: Deal [13 + 3 per level] per second to enemies standing on blight.
-Enhanced Blight: Allied Heroes standing on blight have their mana and health regeneration increased by 25% and healing effects increased by 10%.
-Bigger Flask: Increased the range by 25% and the radius of Toss Potion by 50%.
-Smash Vial: Blood Vial also infects enemies around the initial target.
Tier6 (16)
-Perfect Blight: Double blights effects (base effects only) and Heroes now also take 10% increased damage.
-Improved Unholy Frenzy: Also increases Basic Attack damage.
-Will of the Forsaken: Activate to become Unstoppable and gain 30% movement speed for 3 seconds.
-Hardened Focus
Tier7 (20)
-Improved Mixture: Initially heals [258 + 18 per level], 25% increased range and radius.
-Blight Barrel: Increase damage by 20%, now also spreads blight in its damage radius.
-Storm Shield
-Rewind
Personally, I like ability-driven heroes, like diablo. They have a rhythm to them, a set of steps that you follow to do the best with that hero. Yeah, you can mix it up a little, but that rhythm still exists. This concept seems to have that rhythm. Only thing I'd change is for Unholy Frenzy, switch out the ability damage for attack speed and lengthen the cooldown a bit. Other than that and a few tweaks here and there, I think you have a fine concept.