Vadrok, Apex of Thunder
Legendary Creature — Elemental Dinosaur Cat
Mutate 
(If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flying, first strike
Whenever this creature mutates, you may cast target noncreature card with mana value 3 or less from your graveyard without paying its mana cost.
Zack Stella
Rulings
If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, such as that of Swallow Whole, those must be paid to cast the card.
(wotc • 2020-04-17)
If you wish to cast the target card, you do so as part of the resolution of Vadrok’s ability. You can’t wait to cast it later. Timing permissions based on a card’s type are ignored.
(wotc • 2020-04-17)
If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
(wotc • 2020-04-17)
Rulings
If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, such as that of Swallow Whole, those must be paid to cast the card.
(wotc • 2020-04-17)
If you wish to cast the target card, you do so as part of the resolution of Vadrok’s ability. You can’t wait to cast it later. Timing permissions based on a card’s type are ignored.
(wotc • 2020-04-17)
If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
(wotc • 2020-04-17)