[Master Shake]

Units that want to move have to be assigned to a commander. If you assiniate that commander (which happens before movement), the units assinged to it will not move. But everyone else will move normally. If you kill all commanders, obviously no units will move.

If you are attacking the province where your assasins are working (assuming your enemy does not order his units to move), any enemy units assigned to an assasinated commander will be in the default position (center of their side of the battlefield, just like units not yet assigned to a commander.