If players acquire the transformation card, they will be able to transform into almost every monster in game -- including bosses. When in altered form, players then get some of that monster's special abilities to use on top of their own. Want to be a wolf? Go kill a wolf. The principle flows through to PvP as well; killing another player will net the victor some of the loser's skills. Choosing which form to fight in also becomes a matter of strategy, as some monsters' powers change depending on time of day, location, etc.
Want to get a gander at some of the available forms players can turn into? Catch a glimpse in the trailer after the break.