It doesn't instantly deal harm; it deals damage when the opponent begins its turn. A 5-foot cube of spinning daggers is formed, but the cloud is static, not moving. Anyone who joins or begins their turn in the cloud takes 4d4 damage just immediately. No roll to hit and no saving throw, much like the way Magic Missile does harm with no throw to hit or save. If you place it on a square with an opponent in it, no saving throw, it causes harm instantly.