Splinter, the Mentor is a mythic Legendary Creature — Mutant Ninja Rat card from Teenage Mutant Ninja Turtles Eternal (TMC), collector number 3. Current market price: $2.14 (foil $0.53), TCGplayer market prices via Scryfall.
Menace Whenever Splinter or another nontoken creature you control leaves the battlefield, create a Mutagen token. Partner—Character select (You can have two commanders if both have this ability.)