Hearthpwn

Mistermath's Card Ratings for Voyage to the Sunken City:

7/10

The fire Lady Naz'jar casts Explosive Shot, the frost Lady Naz'jar casts Heavy Plate, and the arcane one reduces the cost of spells in hand by 1 (an effect not on an individual card yet, but is maybe comparable to Palm Reading). Of the three, fire is the clear winner, and you really want to be using Lady Naz'jar with cards like Fire Sale and First Flame to maintain board control. Note that this is also pretty flexible; you can swap between all the forms as long as this is in your hand. Also... Goddamnit Blizzard, STOP PRINTING Firelands Portals WITH UPSIDE AND DISCOUNTS ON MANA!

Better than the "cast three spells" cards at least....

Arcane = Battle Cry: Reduce the Cost of Spells in your hand by (1).
Fire = Battle Cry: Deal 5 Damage to an enemy minion and 2 to adjacent minions.
Frost = Battle Cry: Gain 8 Armor.

So is it permanent transformation after one spell or whenever one is played?

I would think based on the wording that the last spell from one of these schools before playing the card counts.

Transforms into what

Seems like it'll transform based on whether you played an arcane, fire or frost since the transformations match up with those schools nicely.  Let's be honest though, it won't surprise anyone if it's rng based even though it doesn't say transform randomly.