While North-Wind Herald is free, it has no influence requirements.
On the turn you play North-Wind Herald for free, playing Unstable Form on it will transform the Herald into a 1-cost unit. However, it will resume being a 6-cost unit the turn after it's played. Playing the Unstable Form as your first spell played will then transform it into a 7-cost unit.
Sand Warrior may be the fifth card in this cycle. It is a Time card with no power cost, with the 'condition' of having 3 or more Time influence. However, it does not share the similar text of the other cards.