Mister Cheddar, Cheese Sliver (-) - Un-Known Event Playtest Cards

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."