The full rule reads: If a player is casting a spell that has an X in its mana cost, the value of X isn't defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0
BoP is the best and Arbor the worst but situationally that can be completely reversed
An Effect Monster Special Summoned this way has its effect negated
Magician of Dark Illusion Magician of Dark Illusion is a cool spin-off of Dark Magician, and is kind of the complete opposite of Dark Magician
Its a serious-looking Demon, wreathed in mist, wielding a glowing sword
We're all quite used to this effect on digital card games, especially in Hearthstone where cards that shuffle cards into the opponent's deck are fairly commonplace