The Powerful Dragon is a rare Legendary Creature — Dragon card from Unknown Event (UNK), collector number RR06b. Current market price: $29.00, TCGplayer market prices via Scryfall.
Flying Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.) Whenever one or more +1/+1 counters are put on a creature you control, exile the top card of your library. Until end of turn, you may play that card.