Casting Eidolon of the Great Revel doesn't cause its own ability to trigger. It must be on the battlefield when the spell is cast for its ability to trigger.
16-03-2018
wotc
The amount of mana spent to cast a spell doesn't change its mana value. For example, a card cast with its morph ability has a mana value of 0 even though {3} was paid.
16-03-2018
wotc
The mana value of a spell with {X} in its mana cost includes the value chosen for X.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.
Cyril Van Der Haegen2018-03-16
