Cheapest priced copy (Commander Masters): $0.39 at Card Kingdom, near mint, their price list as of .
Aryel, Knight of Windgrace 
Legendary Creature — Human Knight · 4/4 · Rare · March of the Machine Commander · #318
Vigilance
{2}{W}, {T}: Create a 2/2 white Knight creature token with vigilance.
{B}, {T}, Tap X untapped Knights you control: Destroy target creature with power X or less.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.39 to $1.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Masters | 336 | Uncommon | $0.39 | $0.79 |
| Dominaria | 192 | Rare | $0.49 | $0.49 |
| March of the Machine Commander | 318 | Rare | $0.49 | — |
| Dominaria Promos | 192s | Rare | — | $1.79 |
4 rulings from Wizards of the Coast
- 27 April 2018Once 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.
- 27 April 2018If 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.
- 27 April 2018Tapping Aryel to activate either of its abilities while it's attacking doesn't remove it from combat.
- 27 April 2018You 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.