The Misty Stepper (Playtest Card)

'You may have The Misty Stepper enter as a copy of a creature you control, except its name is The Misty Stepper and it has its other abilities.
The Misty Stepper can't be blocked.
Spells you cast have split second. (As long as that spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities.)