Eidolon of the Great Revel and spells with Delve have a bit of a love/hate relationship. As the controller of an Eidolon, you love to cast Delve spells yourself, and hate it when you see your opponents cast them.
Well, this is because of the way Delve works. No matter how many cards you exile for the Delve ability, the Converted Mana Cost (CMC) of the spell being cast will stay the same. A spell’s CMC is set by the mana cost printed in the top right corner, then at the same time as paying the costs, for each generic mana in the spells total cost, you may choose to exile a card from your graveyard instead of paying that mana. This will not change the CMC. So that Treasure Cruise you paid U and exiled 7 cards from your graveyard to cast will still have a CMC of 8, which is far above the “3 or less” that the Eidolon cares about.
Today’s Rules Tip written by Angela Schabauer