Mister Cheddar, Cheese Sliver is a rare Legendary Creature — Rat Sliver card from Unknown Event (UNK), collector number RB05a. Current market price: $52.53, TCGplayer market prices via Scryfall.
Rats and Slivers you control are Rat Slivers in addition to their other types and have ratmanship. (They can't be blocked except by Cats, Rats, or other creatures with ratmanship.) As long as Mister Cheddar is your commander, Sliver cards in your deck have "Your deck may contain any number of this card."