April O'Neil, Human Element is a rare Legendary Creature — Human Detective card from Teenage Mutant Ninja Turtles Eternal (TMC), collector number 107. Current market price: $3.18, TCGplayer market prices via Scryfall.
Whenever a player casts an artifact, instant, or sorcery spell, you create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")