Cheapest priced copy (March of the Machine: The Aftermath): $0.79 at Card Kingdom, near mint, their price list as of .
Ayara's Oathsworn 
Creature — Human Knight · 2/2 · Rare · March of the Machine: The Aftermath · #61
Menace
Whenever this creature deals combat damage to a player, if it has fewer than four +1/+1 counters on it, put a +1/+1 counter on it. Then if it has exactly four +1/+1 counters on it, search your library for a card, put it into your hand, then shuffle.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.79 to $4.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine: The Aftermath | 11 | Rare | $0.79 | $0.79 |
| March of the Machine: The Aftermath | 61 | Rare | $2.99 | $0.79 |
| March of the Machine: The Aftermath | 111 | Rare | — | — |
| March of the Machine: The Aftermath | 157 | Rare | $2.29 | $0.99 |
| March of the Machine: The Aftermath | 195 | Rare | — | $4.99 |
3 rulings from Wizards of the Coast
- 12 May 2023If the triggered ability does trigger, that ability will check Ayara's Oathsworn again as it tries to resolve. If Ayara's Oathsworn has four or more +1/+1 counters on it at that time, the ability won't resolve. None of its effects will happen. You won't put a +1/+1 counter on Ayara's Oathsworn and you won't search for a card.
- 12 May 2023If Ayara's Oathsworn has four or more +1/+1 counters on it as it deals combat damage to a player, its ability won't trigger at all. None of its effects will happen. You won't put a +1/+1 counter on Ayara's Oathsworn and you won't search for a card.
- 12 May 2023If Ayara's Oathsworn isn't on the battlefield as its ability tries to resolve, you won't search for a card, even if Ayara's Oathsworn had three +1/+1 counters on it before if left the battlefield, because it won't be around to have the fourth +1/+1 counter put on it.