Vivien, Champion of the Wilds
You may cast creature spells as though they had flash. +1: Until your next turn, up to one target creature gains vigilance and reach. −2: Look at the top three cards of your library. Exile one face down and put the rest on the bottom of your library in any order. For as long as it remains exiled, you may cast it if it's a creature spell.
Rulings (3)
May 3, 2019 · You must follow any applicable timing permissions and restrictions if you cast the exiled card. This means that if Vivien leaves the battlefield, you can't cast the creature spell as though it had flash unless another effect allows you to do so.
May 3, 2019 · Once you cast the creature card, it leaves exile. You can't cast it again if that card becomes exiled again.
May 3, 2019 · While resolving Vivien's last ability, you must exile one of the three cards, even if none of them are creature cards. You may look at the exiled card at any time, even if it's not a creature card.