Molten-Core Maestro is a rare Creature — Goblin Bard card from Secrets of Strixhaven (SOS), collector number 335. Current market price: $0.79 (foil $0.81), TCGplayer market prices via Scryfall.
Menace Opus — Whenever you cast an instant or sorcery spell, put a +1/+1 counter on this creature. If five or more mana was spent to cast that spell, add an amount of {R} equal to this creature's power.