Transform was a mechanic added in Innistrad and Dark Ascension, and was the introduction of double-faced cards.
The gimmick behind these cards is that they have no Magic: The Gathering card back.
They instead have two faces – a casting side, that represents the card in all zones. The casting cost, CMC, color, etc are all dependent on the regular face of the card,indicated by a Sun in upper left hand corner.
Mayor of Avabruck for example is a human lord that gives humans +1/+1 – but if no spells were cast this turn, he flips, which is where transform comes in.
Creatures with transform have several different ways to flip. Werewolves in particular have the most transform cards. Their human sides flip (and have to flip) the upkeep after any turn that no spells are cast.
Howlpack Alpha on the other hand is a Werewolf and Wolf lord – he gives +1/+1 to each creature of those types, and also makes 2/2 green wolf tokens during the end step. Similar to it’s human side, the werewolf flips the upkeep after each turn where 2 or more spells are cast.
The mechanic here is obviously very flavorful – if no actions are taken, a human has no choice but to become their darker, werewolf half, and the werewolf side takes a considerable effort from either player to flip back.
However, Transforming isn’t reserved for just the werewolves.
From this card, Screeching Bat, we can see that Vampires don’t have to transform, and even these transformations aren’t one way. For 2BB, you can flip between a 2/2 flier or a 5/5 on the ground during the upkeep, which is always a nice option, between power and evasion. Flavorfully, it matches the old trope of vampires being able to transform into bats at will – which is actually one not often touched on in MTG.
There are plenty of other transform creatures, but the essence is this – if a condition is fulfilled or payed for, you get to transform the creature. Some, like the above, go back and forth.
Things like Elbrus, the Binding Blade into Withengar Unbound however can’t be reversed.
Now, will we see transform again? Well, it’s pretty likely, but it will have to flavorfully fit the block, or the individual card if they make a one-of in some set like the Commander decks or draft sets like Conspiracy.
Until next time,