Ayara's Oathsworn is a rare Creature — Human Knight card from March of the Machine: The Aftermath (MAT), collector number 157. Current market price: $0.95 (foil $1.33), TCGplayer market prices via Scryfall.
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.