Whenever Karn blocks or becomes blocked, it gets -4/+4 until end of turn.
: Target noncreature artifact becomes an artifact creature with power and toughness each equal to its mana value until end of turn.
You may have Sakashima the Impostor enter as a copy of any creature on the battlefield, except its name is Sakashima the Impostor, it's legendary in addition to its other types, and it has ": Return Sakashima the Impostor to its owner's hand at the beginning of the next end step."