This is the Arcane Giant HS Card being released as part of the new hearthstone adventure One Night in Karazhan.
The card reads: Costs (1) less for each spell you’ve cast this game.
I like this card. We have 8/8 grants that reduce in cost each hero power, each card in hand, each card in opponent’s hand, damage taken, and now for spells cast.
This is one of the more playable giants for sure. In Tempo Mage this card can end up costing 0 mana. Whether it fits in with the deck is a separate consideration, but you can certainly get good value from it.