Arcades Sabboth, the Eldest

Flying, hexproof
When Arcades Sabboth, the Eldest enters the battlefield, choose a creature type. It has that chosen type in addition to its other creatures and types.
At the beginning of your upkeep, if you have ten or more creatures of the chosen type, creatures you control lose all other abilities, and become 4/4 white Dragon creatures with flying in addition to its other types, until end of turn.