Bestow mechanics, copied from a twitter post from a wizards guy.
The Comp Rules haven’t been updated yet, but I’m about 99.5% sure these will be the final rules for bestow.
702.102. Bestow
702.102a Bestow represents two static abilities, one that functions while the card with bestow is on the stack and another that functions both while it’s on stack and while it’s on the battlefield. “Bestow [cost]” means “You may cast this card by paying [cost] rather than its mana cost.” and “If you chose to pay this spell’s bestow cost, it becomes an Aura enchantment and gains enchant creature. These effects last until one of two things happens: this spell has an illegal target as it resolves or the permanent this spell becomes, becomes unattached.” Paying a card’s bestow cost follows the rules for paying alternative costs in rules 601.2b and 601.2e–g.
702.102b If a spell’s controller chooses to pay its bestow cost, that player chooses a legal target for that Aura spell as defined by its enchant creature ability and rule 601.2c. See also rule 303.4.
702.102c A spell’s controller can’t choose to pay its bestow cost unless that player can choose a legal target for that spell after it becomes an Aura spell.
702.102d As an Aura spell with bestow begins resolving, if its target is illegal, the effect making it an Aura spell ends. It continues resolving as a creature spell and will be put onto the battlefield under the control of the spell’s controller. This is an exception to rule 608.3a.
This means a couple things. 1) you can counter a "bestow" spell on the stack with negate (it does _not_ count as a creature). 2) if the target becomes illegal it still _resolves_ as a creature. You never get the chance to counter it with something like essence scatter. 3) CMC is what is on top of the card in any zone *except* when it's being cast as an aura. 3)the bestow cost does not count towards devotion (should it have different mana symbols than the CMC of the card). 4)when/if the attached creature dies it then becomes the creature type printed on the card.
Educate your friends. This is bound to come up.
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