So, if you've watched the original Yu-Gi-Oh series, you've most likely heard of the Change Of Heart and Monster Reborn cards. They may not be the most iconic cards of the series, but they do play their parts. They are actually quite useful cards, since the Change Of Heart card will let you control an opponent's monster for one turn, and the Monster Reborn card lets you bring back a monster from either your graveyard or your opponent's graveyard in attack or face-up defense position. However, these cards have become forbidden, which means they can't be used in your main deck, extra deck, or side deck. But, why? Although I may not know the exact answer, it may be because it gives the other person an unfair advantage. For example, a whole duel could be turned around if a strong monster was brought back with a Monster Reborn card, or if your opponent controlled your strong monster to use for themselves for one turn. However, isn't this kind of strategy what dueling is all about? At least, that's my opinion on it. Yu-Gi-Oh is a game of skill and wits, after all.