Zerug seems far too weak to me. Moroes saw no play, and I think 2/2 with rush much later is actually weaker than a 1/1 immediately. I think the card could say "whenever a minion dies" and it wouldn't be overpowered.
ikarigami: I do think that the previously suggested "First spell a turn" suggestion would help rebalance this. My other suggestion would be that this weapon loses durability to give a Spell extra damage equal to it's cost, then probably drop it's durability to 9 (If you make it conditional then we don't have 20 damage Pyroblast shenanigans). This keeps the flavor for Fireball whilst removing the issue of 0-cost spells and also making Arcane Missiles not an insta kill. Like the concept overall
Fair points. Toned it down a bit to avoid abuse while still letting it keep the Fireball to Pyroblast flavor without resulting in 20-damage Pyroblasts or 20-damage Arcane Missile spam.