Some transformations take place as a state-based action and can't be responded to, as we see with daybound/nightbound werewolves
My preferred method for using Nevinyrral to create an instant army is finding a way to give it Flash through effects like Leyline of Anticipation or Vedalken Orrery so you can cast it on someone elses turn, then bring it out in response to another players board wipe
Her set symbol (a chocobo, which you can see by the right of her type line) indicates she's from Final Fantasy Commander, so very likely a card we'll find in the Limit Break Commander precon, headed by Cloud, Ex-SOLDIER
Landfall isn't the only mechanic in play, either, as earthbend is also extremely prominent
After escaping from the group of Ghosts, the player meets Primo, who combines with his Duel Runner and challenges the player, while the player's companions step up to take care of the remaining Ghosts