Rulings
Your opponents may cast spells in response to the last ability of Ranger-Captain of Eos. The ability won’t affect those spells and it won’t affect spells that those players cast before you activated it. (In other words, the ability can’t be used to counter a spell.)
(wotc • 2019-06-14)
If a card in a player’s library has {X} in its mana cost, X is considered to be 0.
(wotc • 2019-06-14)
Rulings
Your opponents may cast spells in response to the last ability of Ranger-Captain of Eos. The ability won’t affect those spells and it won’t affect spells that those players cast before you activated it. (In other words, the ability can’t be used to counter a spell.)
(wotc • 2019-06-14)
If a card in a player’s library has {X} in its mana cost, X is considered to be 0.
(wotc • 2019-06-14)