Free if you played two spells this turn.
North-Wind Herald is a Unit.
How to Get Edit
This card can also be obtained from Card Packs. Diamond Chests and Card Packs can give Premium versions of this card.
If North-Wind Herald is played for free, on the turn you play it, playing Unstable Form on it, will transform the Herald into a 1-cost unit. However, if it's the turn after it's played, it will resume being a 6-cost unit, and playing Unstable Form as the first spell you play that turn will transform it into a 7-cost unit.