Aryel, Knight of Windgrace

• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• WWizards of the Coast2018-04-27
  Official Ruling
  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.
• WWizards of the Coast2018-04-27
  Official Ruling
  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.
• WWizards of the Coast2018-04-27
  Official Ruling
  Tapping Aryel to activate either of its abilities while it's attacking doesn't remove it from combat.
• WWizards of the Coast2018-04-27
  Official Ruling
  You can tap any untapped Knights you control, including ones you haven't controlled continuously since the beginning of your most recent turn, to pay the cost of Aryel's last ability. However, you must have controlled Aryel continuously since the beginning of your most recent turn in order to use either of its activated abilities.

Rulings via Scryfall