Once you announce that you're activating Aryel's last ability, no player may take other actions until the ability's been paid for. Notably, players can't try to make the value of X invalid by removing or tapping your Knights.
Aryel, Knight of Windgrace
If the target creature's power is greater than X as Aryel's last ability tries to resolve, the ability doesn't resolve. You can't tap extra Knights once the ability has been activated.
Tapping Aryel to activate either of its abilities while it's attacking doesn't remove it from combat.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Grzegorz Rutkowski
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
