Return to the Sewers is a common Instant card from Teenage Mutant Ninja Turtles (TMT), collector number 52. Current market price: $0.12 (foil $0.18), TCGplayer market prices via Scryfall.
Target creature's owner puts it on their choice of the top or bottom of their library. 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.")