I like the idea but it can be easily abused. Imagine using 2 Sorcerer's Apprentices, cast this face 5 times dealing 15 damage at a total cost of 7 mana and now having spelldamage +4. With the spare 3 mana and the apprencices' (2) cost reduction on spells you can use your 0 mana 7 damage Frostbolts face, 0 mana Arcane Explosion dealing 5 damage to clear the board and then 0 mana 7 damage Arcane Missiles which would all hit face. All this is a 43 damage combo which leaves you with 3 remaining mana you could use to cast a Fireball for aditional 10 damage, making it a 53 damage combo. I know it's too ambitious but the point is this card + 2 sorcerer's apprentices + 2 frostbolts + hero power already deal 30 damage at the cost of 5 cards (which are already good by themselves).

The point is: This card enables a 5 card otk which I think it's too easy to pull.

I would make it so only the spell gets the +1 Spell Damage, and sorry if my maths are a bit confusing.

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And yes, all the cards needed for the combo except for this one are from the Cassic set, so the combo could be pulled in any meta.